staging: drm/imx: add drm plane support

This patch adds support for a drm overlay plane on DI0 using the DP.
In principle, the overlay plane could also be used on DI1, but to switch
the overlay plane between display interfaces, the base planes would have
to be exchanged transparently while both display interfaces are inactive.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>