drm/virtio: send vblank event after crtc updates

virtio_gpu was failing to send vblank events when using the atomic IOCTL
with the DRM_MODE_PAGE_FLIP_EVENT flag set. This patch fixes each and
enables atomic pageflips updates.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>