BACKPORT: drm/exynos: add property for plane zpos

BACKPORT: 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.

BUG=chromium:222980
TEST=Verified that cursor works when using property to set zpos.

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>
(cherry picked from commit 00ae67cf26fad3889e71e3bdbec012b1f938dc0e)

Change-Id: I018b6475916f6bbff416d0313fe0c7c3c68217b8
Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/46957
Reviewed-by: John Sheu <sheu@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>