media: imx: csis: Store pads format separately

As the formats on the sink and source pad might be different store
them separately.

The pad format is used to configure the image width and height in
mipi_csis_system_enable(). As the csis cannot downscale, using the sink
or the source one isn't relevant.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>