drm: rcar_lvds: Fix dual link mode operations

The R-Car LVDS encoder units support dual-link operations by splitting
the pixel output between the primary encoder and the companion encoder.

Currently the companion encoder fails at probe time, causing the
registration of the primary to fail as well, preventing the whole DU unit
from being registered at all.

Fix this by not bailing out from probe with error if the
"renesas,companion" property is not specified.

Fixes: fa440d87 ("drm: rcar-du: lvds: Add support for dual-link mode")
Reported-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

drivers/gpu/drm/rcar-du/rcar_lvds.c

@@ -673,10 +673,8 @@ static int rcar_lvds_parse_dt_companion(struct rcar_lvds *lvds)
 
 	/* Locate the companion LVDS encoder for dual-link operation, if any. */
 	companion = of_parse_phandle(dev->of_node, "renesas,companion", 0);
-	if (!companion) {
-		dev_err(dev, "Companion LVDS encoder not found\n");
-		return -ENXIO;
-	}
+	if (!companion)
+		return 0;
 
 	/*
 	 * Sanity check: the companion encoder must have the same compatible