staging: drm/imx: Enable DRM PRIME support

Lets the IPU driver make use of the PRIME functionality introduced
by the "drm: GEM CMA: Add DRM PRIME support" patch.

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