xf86: don't add gpus from udev if autoAddGPU is set

At startup the server wasn't adding devices, but nothing
was blocking hotplug devices by the look of it.

bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91388
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>

hw/xfree86/common/xf86platformBus.c

@@ -477,6 +477,9 @@ xf86platformAddDevice(int index)
     screenLayoutPtr layout;
     static const char *hotplug_driver_name = "modesetting";
 
+    if (!xf86Info.autoAddGPU)
+        return -1;
+
     /* force load the driver for now */
     xf86LoadOneModule(hotplug_driver_name, NULL);