drm/nouveau/kms: Remove redundant zpos initialisation

The nouveau KMS driver will call drm_plane_create_zpos_property() with
an init value depending on the plane purpose.

Since the initial value wasn't carried over in the state, the driver had
to set it again in nv50_wndw_reset(). However, the helpers have been
adjusted to set it properly at reset, so this is not needed anymore.

Cc: nouveau@lists.freedesktop.org
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221095918.18763-14-maxime@cerno.tech