This is somewhat similar to the "Model S does not have a CD player" discussions that bubbled up a lot around 2-3 years ago and seem to have subsided.
In the Model S, Tesla installed the the absolutely most inexpensive audio source solutions they could - which is, two USB sockets (which you could say were already going to be included) and some software built into the car (that uses the definitely-already-included internet hardware). Pretty cheap! In fact you could say that the audio source solutions cost Tesla NOTHING on the assembly line. Only back in the software department.
I see no reason why they would deviate from this tactic in the Model X. I think they will still attempt to spend NOTHING during manufacture. The solution will be for you to bring your own movies into the car.
Streaming movies into the car would use massive amounts of the internet data Tesla that currently provides to us at no cost. (It would cost Tesla a fortune) If Tesla introduces a monthly internet data package, where we pay a monthly fee to stream any amount of data into the car via its LTE connection and offer it via wi-fi to devices brought into the car by occupants, that could be a solution.
I was thinking about this internet data package recently while pondering the long-talked-about SDK for the car. If you installed a 3rd-party app into your car that uses tons of internet data, Tesla won't want to be paying for that data!