drm: rcar-du: lvds: Don't fail probe if output is not connected on D3/E3

On the D3 and E3 SoCs the LVDS encoder has an extended internal PLL and
supplies a clock to the DU. That clock is used not only for the LVDS
outputs but also for the DPAD output. The LVDS encoder thus needs to be
available to the DU even when its output is disabled. Don't fail probe
in that case on D3 and E3.

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