https://bugs.freedesktop.org/show_bug.cgi?id=74927
--- Comment #4 from Alex Deucher agd5f@yahoo.com --- Make sure your kernel has this patch: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=22...
You can also try disabling tiling in your xorg config:
Section "Device" Identifier "Radeon" Driver "radeon" Option "ColorTiling" "true" Option "ColorTiling2D" "true" EndSection