drm: omapdrm: Move color modes feature to dispc_features structure

The supported_color_modes is a dispc feature. Move it from the
omap_dss_features structure to the dispc_features structure.

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