drm/omap: dss: Move DSS mgr ops and private data to dss_device

The DSS manager ops and private data pointer are specific to a DSS
instance. Store them in the dss_device structure instead of global
variable.

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>