backlight: Fix external uses of backlight internal semaphore

backlight_device->sem has a very specific use as documented in the
header file. The external users of this are using it for a different
reason, to serialise access to the update_status() method.

backlight users were supposed to implement their own internal
serialisation of update_status() if needed but everyone is doing
things differently and incorrectly. Therefore add a global mutex to
take care of serialisation for everyone, once and for all.

Locking for get_brightness remains optional since most users don't
need it.

Also update the lcd class in a similar way.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>

drivers/video/backlight/lcd.c

@@ -198,6 +198,7 @@ struct lcd_device *lcd_device_register(const char *name, void *devdata,
 		return ERR_PTR(-ENOMEM);
 
 	init_MUTEX(&new_ld->sem);
+	mutex_init(&new_ld->update_lock);
 	new_ld->props = lp;
 	memset(&new_ld->class_dev, 0, sizeof(new_ld->class_dev));
 	new_ld->class_dev.class = &lcd_class;