Setting resolution of external monitor

[Oliver Block]

Dear list,

I have a problem setting up the resolution of my external monitor
connected to the VGA output of my laptop. The graphics controller is:

01:00.0 VGA compatible controller: ATI Technologies Inc Radeon Mobility
X1400

The laptop display has a native resolution of 1680x1050. The external
screen as a (maximal) resolution of 1920x1080.

I would like to be able to change the resolution to 1920x1080 and to see
just a part of this on the laptop display.

But xrandr does not let me do this:

$ xrandr 
Screen 0: minimum 320 x 200, current 1680 x 1050, maximum 1920 x 1080
VGA_1 connected 1680x1050+0+0 478mm x 269mm
   1680x1050      60.0*    59.9  
   1400x1050      74.8     70.0     60.0  
   1280x1024      75.0     59.9     60.0  
   1440x900       74.9     60.2     60.0  
   1280x960       60.0     59.9  
   1280x800       60.0  
   1152x864       75.0     74.9  
   1280x768       60.0  
   1024x768       75.1     75.0     70.1     60.0  
   832x624        74.6  
   800x600        72.2     75.0     60.3     56.2  
   640x480        75.0     72.8     72.8     75.0     66.7     60.0      59.9  
   720x400        70.1  
PANEL connected 1680x1050+0+0 331mm x 207mm
   1680x1050      60.1*+
TV_SVIDEO disconnected
DVI-D_1 disconnected

You can find my xorg.conf here: http://pastebin.com/m4503905b
The maximal resolution you see at the first line would be 1680x1050 if I
would not specify "Virtual 1920 1080" in xorg.conf.

You also find Xorg.0.log here: http://pastebin.com/m7828c9eb
Please note that there is a Modeline for the maximal resolution (1920x1080)
of the external monitor (PL2209HD).

Can someone explain me why I cannot select 1920x1080 as a resolution?
Any hints what to do?

Thank you very much!

Kind regards,
Oliver