i965: Implement brw_bo_map_unsynchronized() with MAP_ASYNC

i965: Implement brw_bo_map_unsynchronized() with MAP_ASYNC

This way we can let brw_bo_map() choose the best mapping type.

Part of the patch inlines map_gtt() into brw_bo_map_gtt() (and removes
map_gtt()). brw_bo_map_gtt() just wrapped map_gtt() with locking and a
call to set_domain(). map_gtt() is called by brw_bo_map_unsynchronized()
to avoid the call to set_domain(). With the MAP_ASYNC flag, we now have
the same behavior previously provided by brw_bo_map_unsynchronized().

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>