media: rkvdec: Extract rkvdec_fill_decoded_pixfmt into helper

Extract call to v4l2_fill_pixfmt_mp() and ajusting of sizeimage into a
helper. Replace current code with a call to the new helper.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Tested-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Tested-by: Christopher Obbard <chris.obbard@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>