drm: rcar-du: Disable alpha blending for DU planes used with VSP

When the input to a DU channel comes from a VSP, the DU doesn't perform
any blending operation. Select XRGB8888 instead of ARGB8888 to ensure
that the corresponding registers don't get written with invalid values.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>

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

@@ -73,7 +73,7 @@ void rcar_du_vsp_enable(struct rcar_du_crtc *crtc)
 			.src.y2 = mode->vdisplay << 16,
 			.zpos = 0,
 		},
-		.format = rcar_du_format_info(DRM_FORMAT_ARGB8888),
+		.format = rcar_du_format_info(DRM_FORMAT_XRGB8888),
 		.source = RCAR_DU_PLANE_VSPD1,
 		.colorkey = 0,
 	};