drm/rcar: plane: Remove redundant zpos initialisation

The rcar-du KMS driver will call drm_plane_create_zpos_property() with an
init value depending on the plane type.

Since the initial value wasn't carried over in the state, the driver had
to set it again in rcar_du_plane_reset() and rcar_du_vsp_plane_reset().
However, the helpers have been adjusted to set it properly at reset, so
this is not needed anymore.

Cc: linux-renesas-soc@vger.kernel.org
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221095918.18763-16-maxime@cerno.tech