virtio-gpu: fix vblank events

virtio-gpu sends vblank events in virtio_gpu_crtc_atomic_flush, and
because of that it must be called for disabled planes too.  Ask
drm_atomic_helper_commit_planes to do that.

v2: update to use new drm_atomic_helper_commit_planes() API.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>