media: imx: capture: Extract format lookup from __capture_legacy_try_fmt

Extract the format lookup code from __capture_legacy_try_fmt() to a
separate function, it will be reused when validating the video node
format at stream start time.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>