gpu: ipu-v3: remove unused functions

ipu_mbus_code_to_colorspace, ipu_stride_to_bytes, and
ipu_pixelformat_is_planar are unused. Remove them.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>