drm/exynos: extend vblank off delay time.

some platform could be entering to sleep after short time once lcd panel off
but before that vblank could be off by vblank off delay feature. at that time,
vblank doesn't have the pair between vblank_get/put. so this path makes vblank
off delay to have enough.

Signed-off-by: Inki Dae <inki.dae@samsung.com>