drm/nouveau: fix semaphore dmabuf obj

Fixes some dmabuf object errors on nv50 chipset and below.

Cc: stable@vger.kernel.org [3.7+]
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>