[intel] Move bufmgr back to context instead of screen, fixing glthreads.

[intel] Move bufmgr back to context instead of screen, fixing glthreads.

Putting the bufmgr in the screen is not thread-safe since the emit_reloc
changes.  It also led to a significant performance hit from pthread usage
for the attempted thread-safety (up to 12% of a cpu spent on refcounting
protection in single-threaded 965).  The motivation had been to allow
multi-context bufmgr sharing in classic mode, but it wasn't worth the cost.