Will be needed for queries and drm event-driven throttling.

As a benefit, they help avoid stale user-space fence handles.
Signed-off-by: NThomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: NJakob Bornecrantz <jakob@vmware.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>