drm/exynos: fixed blending for hdmi graphic layer

Blending for graphic layer 0 of hdmi mixer was not set so video
layer cannot be showed if graphic layer 0 is enabled.
This patch fixes blending values to support blending between
graphic layer 0 and video layer.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>