drm/exynos: Don't blend mixer layer 0

This patch disables blending the mixer's layer 0 onto the background
(solid color). It doesn't make sense to blend this layer by default,
and causes color distortion if the layer is used for arbitrary content.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>