staging: vboxvideo: Stop accessing crtc_state->active

Atomic modesetting drivers should never check crtc_state->active directly,
instead check crtc_state->enable.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>