staging: drm/imx: update TODO file

One of the main things to do to the driver is to support
the common display famework (CDF) to hit mainline. As this will
make some changes to the devicetree bindings necessary it makes
sense to do it before we move out of staging.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>