media: rkvdec: Extract decoded format enumeration into helper

Add a rkvdec_is_valid_fmt() helper that check if a fourcc is a supported
CAPTURE format, and a rkvdec_enum_decoded_fmt() helper that enumerates
valid formats.

This moves current code into helper functions in preparation for adding
CAPTURE format filtering and validation in next patch.

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>