drm/nouveau: allow nouveau_fence_ref() to be a noop

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

diff --cc drivers/gpu/drm/nouveau/nouveau_bo.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nouveau_fence.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nouveau_gem.c
Simple merge