drm/omap: remove CLUT

DSS IP versions 2 and 3 support CLUT modes (color lookup table), but the
driver has never supported those. We still have had some code for CLUT
modes. As the newer DSS IP versions have dropped CLUT support, we might
as well clean up the driver by removing the CLUT related code.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>