drm/nouveau: use the correct fence implementation for nv50

Only compile time tested, noticed nv50_fence_create was never used,
so fix this. This will probably fix vblank on nv50 cards.

Hopefully this is still in time for 3.7 final release.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>