drm/exynos: rename zpos to index

This patch renames zpos entry to index, because in most places it is
used as index for selecting hardware layer/window instead of
configurable layer position. This will later enable to make the zpos
property configurable.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>