ACPI: video: Simplify acpi_video_unregister_backlight()

When acpi_video_register() has not run yet the video_bus_head will be
empty, so there is no need to check the register_count flag first.
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/acpi/acpi_video.c

@@ -2257,14 +2257,10 @@ void acpi_video_unregister_backlight(void)
 {
 	struct acpi_video_bus *video;
 
-	mutex_lock(&register_count_mutex);
-	if (register_count) {
-		mutex_lock(&video_list_lock);
-		list_for_each_entry(video, &video_bus_head, entry)
-			acpi_video_bus_unregister_backlight(video);
-		mutex_unlock(&video_list_lock);
-	}
-	mutex_unlock(&register_count_mutex);
+	mutex_lock(&video_list_lock);
+	list_for_each_entry(video, &video_bus_head, entry)
+		acpi_video_bus_unregister_backlight(video);
+	mutex_unlock(&video_list_lock);
 }
 
 bool acpi_video_handles_brightness_key_presses(void)