[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-dev.c b/drivers/media/video/v4l2-dev.c
index 1898099..498e674 100644
--- a/drivers/media/video/v4l2-dev.c
+++ b/drivers/media/video/v4l2-dev.c
@@ -578,11 +578,9 @@
 			vdev->parent = vdev->v4l2_dev->dev;
 		if (vdev->ctrl_handler == NULL)
 			vdev->ctrl_handler = vdev->v4l2_dev->ctrl_handler;
-		/* If the prio state pointer is NULL, and if the driver doesn't
-		   handle priorities itself, then use the v4l2_device prio
-		   state. */
-		if (vdev->prio == NULL && vdev->ioctl_ops &&
-				vdev->ioctl_ops->vidioc_s_priority == NULL)
+		/* If the prio state pointer is NULL, then use the v4l2_device
+		   prio state. */
+		if (vdev->prio == NULL)
 			vdev->prio = &vdev->v4l2_dev->prio;
 	}