drm/sun4i: layer: Remove redundant zpos initialisation

The sun4i 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 sun4i_backend_layer_reset().
However, the helpers have been adjusted to set it properly at reset, so
this is not needed anymore.

Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-sunxi@lists.linux.dev
Cc: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221095918.18763-18-maxime@cerno.tech