modesetting: Fix cirrus 24bpp breakage

[Adam Jackson]

On Thu, 2018-07-19 at 14:38 +0200, Stefan Dirsch wrote:
> From: Takashi Iwai <tiwai at suse.de>
> 
> The recent rewrite of modesetting driver broke the 24bpp support.
> As typically found on cirrus KMS, it leads to a blank screen, spewing
> the error like:
>   failed to add fb -22
>   (EE) modeset(0): failed to set mode: Invalid argument
> 
> The culript is that the wrong bpp value of the front buffer is passed
> to drmModeAddFB().  Fix it by replacing with the back buffer bpp,
> drmmode->kbpp.
> 
> Signed-off-by: Takashi Iwai <tiwai at suse.de>
> Tested-by: Stefan Dirsch <sndirsch at suse.de>

Merged, thanks:

remote: remote: I: patch #239767 updated using rev d625e16918ef9104863709eb108346464767c444.        
remote: remote: I: 1 patch(es) updated to state Accepted.        
remote: To ssh://kemper.freedesktop.org/git/xorg/xserver
remote:    cdec2b3c1..d625e1691  d625e16918ef9104863709eb108346464767c444 -> master
To ssh://gitlab.freedesktop.org/xorg/xserver
   cdec2b3c1..d625e1691  master -> master

- ajax