media: staging: media: imx: imx7-media-csi: Split imx_media_dev from probe()

Prepare for the decoupling of the imx7-media-csi driver from the
IPUv3-based drivers by moving the imx_media_dev handling from probe()
function to separate functions.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>