i965: Drop bogus pthread_mutex_unlock in map_gtt error path.

The locking was supposed to go away in commit 314647c4c2
(i965: Drop global bufmgr lock from brw_bo_map_* functions.), but
this lone unlock remains.

I'm guessing I messed this up when splitting up Chris's patch.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

src/mesa/drivers/dri/i965/brw_bufmgr.c

@@ -759,7 +759,6 @@ brw_bo_map_gtt(struct brw_context *brw, struct brw_bo *bo, unsigned flags)
       if (ret != 0) {
          DBG("%s:%d: Error preparing buffer map %d (%s): %s .\n",
              __FILE__, __LINE__, bo->gem_handle, bo->name, strerror(errno));
-         pthread_mutex_unlock(&bufmgr->lock);
          return NULL;
       }