Switch to Intel Iris Pro on MacBook Pro under Windows
I know this is not the answer you want to hear, but unfortunately there is no way to disable the discrete GPU and use the integrated graphics, when using Bootcamp.

This has been discussed (to death) on support forums and on the web generally.

I have thought myself in the past, that it would be useful to be able to turn off the discrete GPU just to improve battery life on the laptop. But Apple have not provided a driver capable of switching off the discrete GPU, so there is no way of doing it.

Sorry to be the bringer of bad news - who knows, maybe they will provide that functionality in the future?