drm: rcar-du: Drop LVDS device tree backward compatibility

The rcar-du driver goes to great lengths to preserve device tree
backward compatibility for the LVDS encoders by patching old device
trees at runtime.

The last R-Car Gen2 platform was converted to the new bindings commit
edb0c3af ("ARM: dts: r8a7793: Convert to new LVDS DT bindings"),
in v4.17, and the last RZ/G1 platform converted in commit
6a6a7976 ("ARM: dts: r8a7743: Convert to new LVDS DT bindings"),
in v5.0. Both are older than commit 58256143

 ("clk: renesas:
Remove R-Car Gen2 legacy DT clock support"), in v5.5, which removes
support for legacy bindings for clocks. The LVDS compatibility code is
thus not needed anymore. Drop it.

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