drm/exynos: make wait_for_vblank a manager op

Changelog v2:
remove unnecessay wait_for_vblank call.
- with this patch, wait_for_vblank callback is moved from
  overlay ops to manager ops so it should be removed and
  it doesn't need to wait vblank signal at plane disable.

Changelog v1:
The wait_for_vblank callback is moved from overlay ops to manager ops
of exynos drm driver. Also, the check for DPMS OFF of encoder is
removed before calling wait_for_vblank.

Signed-off-by: Prathyush K <prathyush.k@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>