Comment # 4 on bug 74927 from Alex Deucher

Make sure your kernel has this patch:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=227ae10f17a5f2fd1307b7e582b603ef7bbb7e97

You can also try disabling tiling in your xorg config:

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