drm/exynos: add wait_for_vblank callback interface.

Changelog v2:
fixed comments.

Changelog v1:
this interface can be used to make sure that hardware overlay is disabled
to avoid that memory region is accessed by dma after gem buffer was released.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>