drm/nouveau: implement prime helper unpin function

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

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

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

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