drm/exynos: control blending of mixer graphic layer 0

The mixer graphic layer 0 isn't blended as default by commit
0377f4ed

(drm/exynos: Don't blend mixer
layer 0). But it needs to be blended with graphic layer 0 if video layer
is enabled by vp because video layer is bottom.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>