intel: Remove the struct intel_region reuse hash table.

intel: Remove the struct intel_region reuse hash table.

I don't see any reason for it -- it was introduced with the DRI2
invalidate work by krh in 2010 with no explanation.  I suspect it was
something about wanting the same drm_intel_bo struct underneath multiple
openings of the BO within one process, but that's covered by libdrm at
this point.  As far as the struct region goes, it is not threadsafe, so
multiple contexts sharing a region could have mixed up the map_count and
assertion failed or worse.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>