Comment # 8 on bug 964837 from Stefan Dirsch

I see. X1600/R5XX is about from 2007/2008. At this time 1080p wasn't that
common yet, so the resolution is lacking from the VBIOS. The good thing is that
we think it's a tiling issue. Check 'man radeon' how to disable tiling. Make
use of

  /etc/X11/xorg.conf.d/50-device.conf

Content should look like this:

Section "Device"
  Identifier "Default Device"
  Driver "radeon"
  Option "ColorTiling" "off"
  Option "ColorTiling2D" "off"
EndSection

Restart your Xserver/Xsession after editing this file.