drm/radeon: cope with foreign fences inside the reservation object

Not the whole world is a radeon! :-)

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>