The CUR_OFFSET register only has 27 bits for the cursor offset, so if the EXA
offscreen memory starts behind 128 MB from the start of video RAM, the offset
wraps and the cursor shows random garbage.

I'm going to fix this with a long due cleanup of the radeon HW cursor memory
handling with EXA, reserving it statically instead of putting in the effort of
dealing with the EXA offscreen memory manager for no gain. If someone's looking
for a quick'n'dirty fix, try moving the EXA offscreen memory from the end of
video RAM.

Meanwhile, possible workarounds are (from most to least preferable IMO):

* Reduce the amount of offscreen memory reserved for textures with Option
"FBTexPercent"
* Reduce the amount of mapped video RAM with the VideoRam directive
* Option "SWcursor"