drm/omap: dss: Fix output next device lookup in DT

The DSS core looks up the next device connected to an output by
traversing the OF graph. It currently hardcodes the local port number to
0, which breaks any output with a different port number (SDI on OMAP3
and any DPI output but the first one). Fix this by repurposing the
currently unused of_ports bitmask in omap_dss_device with an of_port
output port number, and use it to traverse the OF graph.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-26-laurent.pinchart@ideasonboard.com