drm/exynos: wait for the completion of pending page flip

This patch fixes the issue that drm_vblank_get() is failed.

The issus occurs when next page flip request is tried
if previous page flip event wasn't completed yet and then
dpms became off.

So this patch make sure that page flip event is completed
before dpms goes to off.

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