drm/omap: dss: Store dss_device pointer in omap_dss_device

Storing the dss_device pointer in the omap_dss_device structure will
allow accessing the dss_device from the dss_mgr API functions.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>