[media] v4l2: use new flag to enable core priority handling

Rather than guess which driver supports core priority handling, require drivers
that do to explicitly set the V4L2_FL_USE_FH_PRIO flag in video_device.

Updated the core prio handling accordingly and set the flag in the three
drivers that do.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/drivers/media/video/v4l2-fh.c b/drivers/media/video/v4l2-fh.c
index 543b3fe..717f71e 100644
--- a/drivers/media/video/v4l2-fh.c
+++ b/drivers/media/video/v4l2-fh.c
@@ -35,7 +35,6 @@
 	INIT_LIST_HEAD(&fh->list);
 	set_bit(V4L2_FL_USES_V4L2_FH, &fh->vdev->flags);
 	fh->prio = V4L2_PRIORITY_UNSET;
-	BUG_ON(vdev->prio == NULL);
 
 	/*
 	 * fh->events only needs to be initialized if the driver
@@ -54,7 +53,8 @@
 {
 	unsigned long flags;
 
-	v4l2_prio_open(fh->vdev->prio, &fh->prio);
+	if (test_bit(V4L2_FL_USE_FH_PRIO, &fh->vdev->flags))
+		v4l2_prio_open(fh->vdev->prio, &fh->prio);
 	spin_lock_irqsave(&fh->vdev->fh_lock, flags);
 	list_add(&fh->list, &fh->vdev->fh_list);
 	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
@@ -82,7 +82,8 @@
 	spin_lock_irqsave(&fh->vdev->fh_lock, flags);
 	list_del_init(&fh->list);
 	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
-	v4l2_prio_close(fh->vdev->prio, fh->prio);
+	if (test_bit(V4L2_FL_USE_FH_PRIO, &fh->vdev->flags))
+		v4l2_prio_close(fh->vdev->prio, fh->prio);
 }
 EXPORT_SYMBOL_GPL(v4l2_fh_del);