[media] cx25821: switch to v4l2_fh, add event and prio handling

It is now possible to remove cx25821_fh and replace it with v4l2_fh,
which in turn makes event handling and core prio handling possible.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>