This patch resolves the issue that refresh rate got low
at extension mode test with fimd and vidi combination.

The problem was because atomic_commit callback waited
for the completion of vblank to gaurantee crtc relevant
registers are updated from shadow registers to real ones.

However, the waiting there is really unnecessary because
page flip operation does already it.
Signed-off-by: Inki Dae <inki.dae@samsung.com>