Comment # 6 on bug 74927 from Alex Deucher

(In reply to comment #5)
> Hello,
> 
> The kernel I was using was 
> Linux ubuntu 3.13.0-8-generic #27-Ubuntu
> 
> That patch you mention made it to #28, I have since tested with the updated
> kernel and the xorg.conf options you mention, neither have resulted in a fix.

Sorry, logic inversion.  try this:

Section "Device"
    Identifier "Radeon"
    Driver "radeon"
    Option "ColorTiling" "false"
    Option "ColorTiling2D" "false"
EndSection