drm/exynos: move hdmi's wait_for_vblank to manager_ops

The wait_for_vblank callback of hdmi and mixer is now moved from
overlay_ops to manager_ops.

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