[intel] Add a driconf option to cache freed buffer objects for reuse.

[intel] Add a driconf option to cache freed buffer objects for reuse.

This is defaulted off as it has potentially large memory costs for a modest
performance gain.  Ideally we will improve DRM performance to the point where
this optimization is not worth the memory cost in any case, or find some
middle ground in caching only limited numbers of certain buffers.  For now,
this provides a modest 4% improvement in openarena on GM965 and 10% in openarena
on GM945.