drm/qxl: notify that the monitor config changed

drm_helper_hpd_irq_event() only notifies when the connector status
changed. However, Spice monitor config can change while the connector is
connected, to support arbitrary resolution. Do an hotplug event if it
wasn't done by drm_helper_hpd_irq_event().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>