drm/qxl: fix disabling extra monitors from client

To disable a monitor, a Spice client sends a monitor config with the
monitor resolution to 0x0.

However, before qxl_crtc_disable() is reached after the hotplug event,
it can happen that another monitor is reconfigured, and
qxl_send_monitors_config() is called with the old config, which will
re-enable the monitor on the client.

Reset config if monitor is found disconnected, during
drm_helper_hpd_irq_event().

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