Well, you put a DVD drive in the laptop, and run software for viewing the DVDs, and you've got your DVD player.
Or, get a card thatt has video in, and feed your external DVD player into that. (I don't know if there are limitations in doing this).
The LCD display in a laptop is meant for integrated use in the laptop. It's not like a VGA monitor that accepts VGA signals. It is interfaced at a different level. Without the laptop, you'd be missing needed electronics.
Every time a question like this comes up, the answer is pretty pessimistic. There's not enough information available about the laptop displays, and even if one had such information it would require work.
Michael