drm/omap: dss: Replace omap_dss_device port number with bitmask

The omap_dss_device port_num field stores the DT port number associated
with the device. The field is used in different ways depending on the
device type:

- For DPI outputs, the port number is used as an identifier of the DPI
instance

- For sources, the port number is used to look up the omap_dss_device by
DT port node

As omap_dss_device instances are only looked up as sources by sinks,
setting the field to the number of the source port works for both use
cases.

However, to enable looking up sinks, we need to record all the ports
associated with an omap_dss_device. Do so by turning the port_num field
into an of_ports bitmask. For DPI outputs the port number is
additionally stored in the dpi_data structure as the output ID.

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>