HDMI_OVERLAY_NUMBER is specific of mixer driver and be used "windows
layer" term in exynos user manaual, so rename it.
Signed-off-by: NJoonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: NInki Dae <inki.dae@samsung.com>