[media] v4l2_prio: move from v4l2-common to v4l2-dev

We are going to move priority handling into the v4l2 core. As a consequence
the v4l2_prio helper functions need to be moved into the core videodev
module as well to prevent circular dependencies.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>