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/macintosh/via-pmu-backlight.c

@@ -166,11 +166,9 @@ void __init pmu_backlight_init()
 				pmu_backlight_data.max_brightness / 15);
 	}
 
-	down(&bd->sem);
 	bd->props->brightness = level;
 	bd->props->power = FB_BLANK_UNBLANK;
-	bd->props->update_status(bd);
-	up(&bd->sem);
+	backlight_update_status(bd);
 
 	mutex_lock(&pmac_backlight_mutex);
 	if (!pmac_backlight)