media: imx: Use get_mbus_config instead of parsing upstream DT endpoints

Stop parsing upstream neighbors' device-tree endpoints to retrieve the
media bus configuration. Instead use the get_mbus_config op and throw an
error if the upstream subdevice does not implement it.

Also drop the corresponding TODO entry and the now unused
imx_media_get_pad_fwnode() function.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Tested-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>