staging: imx-drm: Convert to new ipu_cpmem API

The ipu_cpmem_*() calls now take a channel pointer instead of a
pointer into cpmem for that channel.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>