drm/omap: change supported_modes to an array

enum omap_color_mode is a bitmask, so at the moment we present the
supported color modes as mask. To be able to move to fourccs, we need to
use an array to present the supported color modes.

As a first step towards fourccs, this patch changes the code to use an
array to store the enums.

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