gpu: ipu-cpmem: Add second buffer support to ipu_cpmem_set_image()

Add a second buffer physaddr to struct ipu_image, for double-buffering
support.

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