drm/msm/mdp5: Remove redundant zpos initialisation

The mdp 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 mdp5_plane_reset(). However, the helpers have been
adjusted to set it properly at reset, so this is not needed anymore.

Cc: freedreno@lists.freedesktop.org
Cc: linux-arm-msm@vger.kernel.org
Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221095918.18763-13-maxime@cerno.tech