Integrate the v4l2_prio_state into the core, ready for use.

One struct v4l2_prio_state is added to v4l2_device and a pointer
to a prio state is added to video_device.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>