[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/cx18/cx18-streams.c b/drivers/media/video/cx18/cx18-streams.c
index 2d24856..c6e2ca3 100644
--- a/drivers/media/video/cx18/cx18-streams.c
+++ b/drivers/media/video/cx18/cx18-streams.c
@@ -207,6 +207,7 @@
 	s->video_dev->fops = &cx18_v4l2_enc_fops;
 	s->video_dev->release = video_device_release;
 	s->video_dev->tvnorms = V4L2_STD_ALL;
+	set_bit(V4L2_FL_USE_FH_PRIO, &s->video_dev->flags);
 	cx18_set_funcs(s->video_dev);
 	return 0;
 }