drm/exynos: Merge overlay_ops into manager_ops

This patch merges overlay_ops into manager_ops. In all cases,
overlay_ops is implemented in the same place as manager ops, it doesn't
serve a functional purpose, and doesn't make things more clear.

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