drm/msm/mdp5: switch to standard zpos property

Instead of implemeting zpos property on our own, use standard zpos
property support.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20210525131316.3117809-6-dmitry.baryshkov@linaro.org


Signed-off-by: Rob Clark <robdclark@chromium.org>