media: imx: imx7_mipi_csis: Ensure pads are connected

The CSI-2 receiver requires both its sink pad and its source pad to be
connected through enabled links in order to be started. Ensure this by
setting the MEDIA_PAD_FL_MUST_CONNECT flag on both pads.

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