drm/omap: dss: Acquire next dssdev at probe time

Look up the next dssdev at probe time based on device tree links for all
DSS outputs and encoders. This will be used to reverse the order of the
dssdev connect and disconnect call chains.

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>