drm/omap: fix scaling limits for WB

WB has additional scaling limits when the output color format is one of
the YUV formats. These limits are not handled at the moment, causing
bad scaling and/or NULL dereference crash.

This patchs adds the check so that dispc returns an error for bad
scaling request.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>