media: rcar-vin: Do not enumerate unsupported pixel formats

If a pixel format is not supported by the hardware NULL is returned by
rvin_format_from_pixel() for that fourcc. Verify that the pixel format
is supported using this or skip it when enumerating.

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