drm: rcar-du: Drop leftovers dependencies from Kconfig

Commit 841281fe ("drm: rcar-du: Drop LVDS device tree backward
compatibility") has removed device tree overlay sources used for
backward compatibility with old bindings, but forgot to remove related
dependencies from Kconfig. Fix it.

Fixes: 841281fe

 ("drm: rcar-du: Drop LVDS device tree backward compatibility")
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>