The supported outputs feature is specific to the DSS, move it from the
omap_dss_features structure to the dss driver.

The omap_dss_features structure is now empty and can be removed.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>