[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/include/media/v4l2-dev.h b/include/media/v4l2-dev.h
index 3700127..8266d5a 100644
--- a/include/media/v4l2-dev.h
+++ b/include/media/v4l2-dev.h
@@ -35,7 +35,10 @@
    Drivers can clear this flag if they want to block all future
    device access. It is cleared by video_unregister_device. */
 #define V4L2_FL_REGISTERED	(0)
+/* file->private_data points to struct v4l2_fh */
 #define V4L2_FL_USES_V4L2_FH	(1)
+/* Use the prio field of v4l2_fh for core priority checking */
+#define V4L2_FL_USE_FH_PRIO	(2)
 
 /* Priority helper functions */