drm/nouveau: complain loudly if buffer is pinned during destruction

Shouldn't happen, and we invert the struct_mutex with reservation here,
potentially leading to deadlocks. Once reservations become lockdep annotated,
lockdep will go splat on this.

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