drm/irq: BUG_ON() -> WARN_ON()

Let's make things a bit easier to debug when things go bad (potentially
under console_lock).
Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_irq.c

@@ -1029,7 +1029,8 @@ void drm_vblank_put(struct drm_device *dev, int crtc)
 {
 	struct drm_vblank_crtc *vblank = &dev->vblank[crtc];
 
-	BUG_ON(atomic_read(&vblank->refcount) == 0);
+	if (WARN_ON(atomic_read(&vblank->refcount) == 0))
+		return;
 
 	if (WARN_ON(crtc >= dev->num_crtcs))
 		return;