drm/omap: do not use BUG_ON(!spin_is_locked(x))

spin_is_locked(x) returns always 0 on uniprocessor, triggering BUG() in
omapdrm.

Change it to use assert_spin_locked() to fix the issue.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
diff --git a/drivers/gpu/drm/omapdrm/omap_irq.c b/drivers/gpu/drm/omapdrm/omap_irq.c
index f035d2b..3eb097e 100644
--- a/drivers/gpu/drm/omapdrm/omap_irq.c
+++ b/drivers/gpu/drm/omapdrm/omap_irq.c
@@ -34,7 +34,7 @@
 	struct omap_drm_irq *irq;
 	uint32_t irqmask = priv->vblank_mask;
 
-	BUG_ON(!spin_is_locked(&list_lock));
+	assert_spin_locked(&list_lock);
 
 	list_for_each_entry(irq, &priv->irq_list, node)
 		irqmask |= irq->irqmask;