Enabling the XRandR-Extension
The extension works only
if DGA is disabled. DGA is a part of extmod, so insert the following
into your modules section of your XF86Config-4:
SubSection "extmod"
Option "omit xfree86-dga"
EndSubSection
This forces the extmod module to load without DGA support. Also
very important: Comment out the extmod module itself, otherwise it
will be loaded again, this time with DGA support, thus disabling
XRandR:
# Load "extmod"
Test XRandR with the xrandr command. If it gives you all configured
resolutions to choose, it works.