drm: rcar-du: Skip encoder allocation for LVDS1 in dual-link mode

The rcar-du driver skips registration of the encoder for the LVDS1
output when LVDS is used in dual-link mode, as the LVDS0 and LVDS1 links
are bundled and handled through the LVDS0 output. It however still
allocates the encoder and immediately destroys it, which is pointless.
Skip allocation of the encoder altogether in that case.

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