drm: rcar-du: Fix build failure

Commit 75a07f399cd4 ("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 301a9b8d5456
("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: 75a07f399cd4 ("drm: rcar-du: Zero-out sg_tables when duplicating plane state")
Signed-off-by: Laurent Pinchart <[e-mail]>
Reviewed-by: Kieran Bingham <[e-mail]>
Tested-by: Kieran Bingham <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

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

···
 		return NULL;
 
 	__drm_atomic_helper_plane_duplicate_state(plane, &copy->state);
-	copy->alpha = to_rcar_vsp_plane_state(plane->state)->alpha;
 
 	return &copy->state;
 }