drm: make sure drm_vblank_init() has been called before touching vbl_lock

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/drm_irq.c

@@ -407,13 +407,16 @@ int drm_irq_uninstall(struct drm_device *dev)
 	/*
 	 * Wake up any waiters so they don't hang.
 	 */
+	if (dev->num_crtcs) {
 		spin_lock_irqsave(&dev->vbl_lock, irqflags);
 		for (i = 0; i < dev->num_crtcs; i++) {
 			DRM_WAKEUP(&dev->vbl_queue[i]);
 			dev->vblank_enabled[i] = 0;
-		dev->last_vblank[i] = dev->driver->get_vblank_counter(dev, i);
+			dev->last_vblank[i] =
+				dev->driver->get_vblank_counter(dev, i);
 		}
 		spin_unlock_irqrestore(&dev->vbl_lock, irqflags);
+	}
 
 	if (!irq_enabled)
 		return -EINVAL;