drm/exynos: make sure that overlay data are updated

Changelog v2:
fix a little bit performance issue to previous patch.
- When drm framebuffer is destroyed, make sure that overlay
  data are updated to real hardwrae for all encoders
  instead of waiting for vblank every page flip request.
  For this, it adds a new function,
  exynos_drm_encoder_complete_scanout function.

Changelog v1:
This patch removes wait_for_vblank call from
exynos_drm_encoder_plane_disable function and move it to
exynos_drm_encoder_plane_commit function.

Disabling dma channel to each plane doens't need vblank
signal to update data to real hardware. But updating
overlay data to real hardware does need vblank signal.

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