drm/omap: dss: Add functions to connect and disconnect devices

The omap_dss_device objects model display components and are connected
at runtime to create display pipelines. The connect and disconnect
operations implemented by each component contain lots of duplicate code.
As a first step towards fixing this, create new functions to wrap the
direct calls to those operations and use them.

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>