media: rcar-vin: Merge helpers dealing with powering the parallel subdevice

The two power helpers are now only dealing with the parallel subdevice,
merge them into a single rvin_power_parallel() helper to reduce code
duplication.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>