drm: samsung: allow enabling vblank while entering hibernation

Allow clients to enable vblank when driver is in hibernation. It allows
us to get vblank timestamp early and able report to composer early.

Bug: 194064433
Test: dispaly on/off, AoD in/out, refresh rate switching
Test: chrome, youtube, camera, video play
Change-Id: I49b9dc35b114efd3375f2832ec10846276bef094
Signed-off-by: Midas Chien <midaschieh@google.com>