drm: rcar-du: Add DSI support to rcar_du_output_name

The DSI output names were not added when the DSI pipeline support was
introduced.

Add the correct labels for these outputs, and fix the sort order to
match 'enum rcar_du_output' while we are here.

Fixes: b291fdcf ("drm: rcar-du: Add r8a779a0 device support")
Suggested-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>