drm/omap: dss: Make omap_dss_device_ops optional

As part of the move to drm_bridge ops, the dssdev ops will become empty
for some of the internal encoders. Make them optional in the driver to
allow them to be removed completely, easing the transition.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-28-laurent.pinchart@ideasonboard.com