drm/nouveau/nouveau: Do not BUG_ON(!spin_is_locked()) on UP

On !SMP systems spinlocks do not exist. Thus checking of they
are active will always fail.

Use
  assert_spin_locked(lock);
instead of
  BUG_ON(!spin_is_locked(lock));
to not BUG() on all UP systems.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>