drm/omap: use DRM_FORMAT_* instead of OMAP_DSS_COLOR_*

This patch changes omapdrm to use DRM_FORMAT_* values instead of
OMAP_DSS_COLOR_* enum.

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