drm: rcar-du: Skip LVDS1 output on Gen3 when using dual-link LVDS mode

In dual-link LVDS mode, the LVDS1 encoder is used as a companion for
LVDS0, and both encoders transmit data from DU0. The LVDS1 output of DU1
can't be used in that case, don't create an encoder and connector for
it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>