drm: rcar-du: Fix build failure

Commit 75a07f3 ("drm: rcar-du: Zero-out sg_tables when duplicating
plane state") introduced a reference to the alpha field of struct
rcar_du_vsp_plane_state that got removed in commit 301a9b8
("drm/rcar-du: Convert to the new generic alpha property"). The issue
stems from the merge of the two commits through separate branches and
breaks compilation of the driver. Fix it.

Fixes: 75a07f3 ("drm: rcar-du: Zero-out sg_tables when duplicating plane state")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180515174752.28954-1-laurent.pinchart+renesas@ideasonboard.com

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

@@ -314,7 +314,6 @@ rcar_du_vsp_plane_atomic_duplicate_state(struct drm_plane *plane)
 		return NULL;
 
 	__drm_atomic_helper_plane_duplicate_state(plane, &copy->state);
-	copy->alpha = to_rcar_vsp_plane_state(plane->state)->alpha;
 
 	return &copy->state;
 }