drm/msm: 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>
[robclark: drop stray ')']
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/mdp/mdp_kms.c

@@ -34,7 +34,7 @@ static void update_irq(struct mdp_kms *mdp_kms)
 	struct mdp_irq *irq;
 	uint32_t irqmask = mdp_kms->vblank_mask;
 
-	BUG_ON(!spin_is_locked(&list_lock));
+	assert_spin_locked(&list_lock);
 
 	list_for_each_entry(irq, &mdp_kms->irq_list, node)
 		irqmask |= irq->irqmask;