gpu: ipu-cpmem: add 16-bit grayscale support to ipu_cpmem_set_image

Add the missing offset calculation for 16-bit grayscale images. Since
the IPU only supports capturing greyscale in raw passthrough mode, it
is the same as 16-bit bayer formats.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>