drm/exynos: Fix default value for zpos plane property

The default zpos property for all planes in Exynos DRM was fixed as zero.
Fix this by providing proper value provided by hardware drivers, which
typically matches hardware window number.

Reported-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Fixes: e47726a1 ("drm/exynos: use generic code for managing zpos plane property")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>