media: rkvdec: Move rkvdec_reset_decoded_fmt helper

Move rkvdec_reset_decoded_fmt() and the called rkvdec_reset_fmt() helper
functions in preparation for adding a new caller in an upcoming 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>