drm: rcar-du: Add support for LVDS mode selection

Retrieve the LVDS mode from the panel and configure the LVDS encoder
accordingly. LVDS mode selection is static as LVDS panels can't be
hot-plugged on any of the device supported by the driver. Support for
dynamic mode selection can be implemented in the future when needed.

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