drm/exynos: change default win layer of mixer to zpos 1

drm/exynos: change default win layer of mixer to zpos 1

Exynos tdm backend of tizen needs that default window layer has zpos 1.
Exynos drm mixer driver of this kernel doesn't support to change zpos
priority of win layer by userspace yet, so this patch modifies zpos of
default window layer to 1 in exynos drm mixer driver.

As the zpos of default win layer is to 1, this patch also removes
workaround of commit 68395cb68321 ("drm/exynos: workaround to change
graphic layers priority"), so will revert priority of win layer.

Change-Id: I9f5a2b0c1a6527f8848ee98e80ec953904b0c630
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>