i965: Drop global bufmgr lock from brw_bo_map_* functions.

i965: Drop global bufmgr lock from brw_bo_map_* functions.

After removing the unusuable debugging code in the previous commit, we
can also entirely remove the global mutex around mapping the buffer for
the first time and replace it with a single atomic operation to update
the cache once we retrieve the mmap.

v2 (Ken): Split out from Chris's original commit.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>