linux-phytec-ti

drm/exynos: make zpos property immutable

We already set each plane zpos at init, after that changes to zpos are
not expected. This patch turns zpos into a read-only property so now it is
impossible to set zpos.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>