drm/virtio: track virtual output state

Track whenever an virtual output (crtc) is enabled or disabled.

On atomic updates check for both framebuffer being present and crtc
being enabled to figure whenever the output is active or not.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180813152855.12863-1-kraxel@redhat.com