drm/omap: Simplify OF lookup of DSS devices

Now that the direction of OF graph walk has been reversed, there's no
need to lookup devices by port as we have no sink device connected
through multiple sink ports. Simplify OF lookup of the DSS devices to
look them up by node only.

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