media: rcar-vin: cache video standard

At stream on time the driver should not query the subdevice for which
standard are used. Instead it should be cached when userspace sets the
standard and used at stream on time.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>