gpu: ipu-cpmem: add 8-bit grayscale support to ipu_cpmem_set_image

Add the missing offset calculation for grayscale images. Since the IPU
only supports capturing greyscale in raw passthrough mode, it is the
same as 8-bit bayer formats.

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