drm/nouveau: fix semaphore dmabuf obj

commit 7a7da592cbb22a1d360638dbecc393470c5effe3 upstream.

Fixes some dmabuf object errors on nv50 chipset and below.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>