drm/exynos: add property for plane zpos

The exynos drm driver used a specific ioctl - DRM_EXYNOS_PLANE_SET_ZPOS
to set zpos of plane. It can be substitute to property of plane. This
patch adds a property for plane zpos and removes
DRM_EXYNOS_PLANE_SET_ZPOS ioctl.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>