[RFC] improve memory placement for radeon

[j.glisse at gmail.com]

So as a followup is 2 patch. The first one just stop trying to move
object at each cs ioctl i believe it could be included in 3.7 as it
improve performances (especialy with vram change from userspace).

The second one implement a vram eviction policy. It's a simple one,
buffer used for write operation are more important than buffer used
for read operation. Buffer get evicted from vram only if they haven't
been use in the last 50ms (so in the last few frames) and only if
there is buffer that have been recently use and that could be move
into vram. This is mostly were i believe discussion should be,
what kind of heuristic would work better than tat.

So without first patch and with mesa master xonotic high is at 17fps,
with first patch it goes to 40fps, with second patch it goes to 48fps.

Cheers,
Jerome