gpu: ipu-v3: export ipu_csi_set_downsize

This function will be used by the media drivers and needs to be exported
to allow them to be built as modules.

Reported-by: Russell King <linux@armlinux.org.uk>
Fixes: 867341b9 ("gpu: ipu-v3: add ipu_csi_set_downsize")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>