[media] v4l2: get/set prio using video_dev prio structure

The v4l2_device structure embed a v4l2_prio_state structure used by
default for priority handling, but drivers can override that default by
setting the video_dev prio pointer to a different v4l2_prio_state
instance.

However, the VIDIO_G_PRIORITY and VIDIOC_S_PRIORITY implementations use
the prio state embedded in v4l2_device unconditionally, breaking drivers
that need to override the default. Fix them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>