I've been having the same problem on Arch Linux for a while now with my Radeon
HD 7950 too. My Kabini laptop, which is also radeonsi, isn't affected. Dmesg
spams the following messages repeatedly:

[ 3069.134039] [TTM] Illegal buffer object size
[ 3069.134046] [drm:radeon_gem_object_create] *ERROR* Failed to allocate GEM
object (0, 6, 4096, -22)