drm/atomic: Use KMS VBLANK API

Instead of using the legacy VBLANK API, use the new KMS API. This is
part of an effort to convert all existing users so that the KMS API can
be changed to properly use per-CRTC data.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>