drm: rcar-du: Remove zpos field from rcar_du_vsp_plane_state structure

Since commit 2fc4d838 ("drm: rcar: use generic code for managing
zpos plane property") the rcar-du driver stores the plane zpos in the
drm_plane_state structure. The commit however forgot to remove the zpos
field from the rcar_du_vsp_plane_state structure. Remove it.

Fixes: 2fc4d838 ("drm: rcar: use generic code for managing zpos plane property")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>