drm: rcar-du: dw-hdmi: Set output port number

Report the DT output port number in dw_hdmi_plat_data to connect to the
next bridge in the dw-hdmi driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham@ideasonboard.com> # On V3U
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>