drm/rcar-du: Rework output routing support

Split the output routing specification between SoC-internal data,
specified in the rcar_du_device_info structure, and board data, passed
through platform data.

The DU has 5 possible outputs (DPAD0/1, LVDS0/1, TCON). SoC-internal
output routing data specify which output are valid, which CRTCs can be
connected to the valid outputs, and the type of in-SoC encoder for the
output.

Platform data then specifies external encoders and the output they are
connected to.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>