drm/sti: plane: Remove redundant zpos initialisation

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

Reviewed-by: Alain Volmat <alain.volmat@foss.st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@foss.st.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221095918.18763-17-maxime@cerno.tech