[media] hdpvr: remove hdpvr_fh and just use v4l2_fh

This prepares the driver for priority and control event handling.
This patch also checks for correct streaming ownership and it makes a
small improvement to the encoder_cmd ioctls: always zero 'flags' and
drop the memset of 'raw' as that is already done by the v4l2 core.

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