drm/omap: Move display alias ID to omap_drm_pipeline

The DT bindings for the OMAP DSS allow assigning numerical IDs to
display outputs through display entries in the alias node. The driver
uses this information to sort pipelines according to the order specified
in DT, making it possible for a system to give a priority order to
outputs.

Retrieval of the alias ID is done when initializing display dss devices.
That code will be removed when moving to drm_bridge and drm_panel. Move
retrieval of the alias ID to display pipeline connection time and store
it in the pipeline structure instead to keep the feature.

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>