i965/drm: Delete NULL check in brw_bo_unmap().

I accidentally moved the bo->bufmgr dereference above the NULL check
when cleaning up this code.

While passing NULL to free() is a common pattern...passing NULL to
unmap seems pretty bad.  You really ought to know whether you have
a buffer or not.  We don't want to paper over bugs like that.  So,
just drop the NULL check altogether.

CID: 1405006

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

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

@@ -827,9 +827,6 @@ brw_bo_unmap(struct brw_bo *bo)
    struct brw_bufmgr *bufmgr = bo->bufmgr;
    int ret = 0;
 
-   if (bo == NULL)
-      return 0;
-
    pthread_mutex_lock(&bufmgr->lock);
 
    if (bo->map_count <= 0) {