drm/virtio: fix vblank handling

virtio has its own commit fail function.  Add the
drm_atomic_helper_fake_vblank() call there.

Fixes: 2a735ad3 ("drm/virtio: Remove sending of vblank event")
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20200205065312.15790-1-kraxel@redhat.com