[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.
Showing with 504 additions and 655 deletions