Multi screen and graphics cards failed with radeon driver on debian stretch

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Hello,

On debian previous version (jessie) I managed to use my 3 screen on KDE but since [b]stretch[/b] update, my conf doesn't work anymore.
If anyone could help me?

My configuration
*************************************************
Ecran 1 (DVI) -- Ecran 2 (VGA) -            - Ecran 3 (VGA)
|                            |         |                       |
|____________________________|         |_______________________|
     Card : Radeon X1550 64-bits         Card : Radeon 3000
     Resolution : Screen 1: 1680x1050    Resolution : Both 1280x1024
                  Screen 2: 1024x768
*************************************************
X.Org X Server 1.19.2
Release Date: 2017-03-02
X Protocol Version 11, Revision 0
Build Operating System: Linux 3.16.0-4-amd64 x86_64 Debian
Current Operating System: Linux diablo 3.16.0-4-amd64 #1 SMP Debian 3.16.43-2 (2017-04-30) x86_64
Kernel command line: BOOT_IMAGE=/boot/vmlinuz-3.16.0-4-amd64 root=UUID=baadc727-bd99-4c84-8e26-3b56c8de18d1 ro quiet
Build Date: 03 March 2017  03:14:41PM
xorg-server 2:1.19.2-1 (https://www.debian.org/support) 
Current version of pixman: 0.34.0
*************************************************

What I already tried

I'm freely inspired by this post: https://bbs.archlinux.org/viewtopic.php?id=143690.

Actually the better I managed is to use 2 screens from my  Radeon X1550 64-bits  graphics card with this xorg.conf (I commented one line with driver "radeon")

*************************************************
Section "ServerLayout"
    Identifier  "Layout0"
    Option      "Xinerama" "on"
    Option      "Clone"    "on"
    # You would need one screen for each monitor
    Screen   0  "Screen0" 0 0
    Screen   1  "Screen1"  LeftOf "Screen0"
    Screen   2  "Screen2"  RightOf "Screen0"

EndSection
Section "Device"
    Identifier  "Device0"
# if ucommented X server failed with segmentation fault:
# Driver      "radeon"
    # Actual PCI location of first card/gpu
    BusID       "PCI:2:0:0"
    Screen      0
EndSection
Section "Device"
    Identifier  "Device1"
    Driver      "radeon"
    BusID       "PCI:2:0:1"
    Screen      0
EndSection
Section "Device"
    Identifier  "Device2"
    Driver      "radeon"
    BusID       "PCI:1:5:0"
    Screen      0
EndSection
Section "Screen"
    Identifier  "Screen0"
    Device      "Device0"
    Monitor     "Monitor0"
    DefaultDepth    24
    SubSection "Display"
      Depth       24
      Modes       "1680x1050"
   EndSubSection
EndSection
Section "Screen"
    Identifier  "Screen1"
    Device      "Device1"
    Monitor     "Monitor1"
    DefaultDepth    24
    SubSection "Display"
      Depth       24
      Modes       "1024x768"
   EndSubSection
EndSection
Section "Screen"
    Identifier  "Screen2"
    Device      "Device2"
    Monitor     "Monitor2"
    DefaultDepth    24
    SubSection "Display"
      Depth       24
      Modes       "1280x1024"
    EndSubSection
EndSection
Section "Monitor"
    Identifier     "Monitor0"
EndSection

Section "Monitor"
    Identifier     "Monitor1"
EndSection

Section "Monitor"
    Identifier     "Monitor2"
EndSection

*************************************************

*************************************************
# lspci |grep VGA
01:05.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RS780L [Radeon 3000]
02:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RV505 [Radeon X1550 64-bit]

$ xrandr --listproviders
Providers: number : 2
Provider 0: id: 0x99 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 2 outputs: 3 associated providers: 0 name:ATI Radeon X1550 64-bit @ pci:0000:02:00.0
Provider 1: id: 0x53 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 2 outputs: 2 associated providers: 0 name:ATI Radeon 3000 Graphics @ pci:0000:01:05.0

*************************************************

if I activate xinerama and set "radeon" driver on first graphic card (like I used on Jessie), X server crash with segmentation fault.
If I set "radeon" driver for my 2 graphics cards then I must deactivate xinerama option and I can use only 2 screen from my graphics cards and xrandr  :? 

Actually I enabled xinerama and leave empty "radeon" driver on first graphics cards but I can use only my 2 screen from my first graphics cards (multi head)


Any help will be welcome (or a nice site to obtain help :-).

Thxs
Pascal BASTIEN