How do you stop a dementia patient from drinking so much?

Fill the glass with ice first

I would definitely have a talk with her dr. about it all, but in the meanwhile trade out all the glasses for smaller ones (juice gasses or kid's glasses). Sometimes just the idea of finishing a glass of water is enough, and if you use small glasses (you too, so there are not larger ones available) she will feel like she has all her water! Just an idea. But for sure talk to the dr!

Are you the one preparing her drinks for her, or is she able to get them herself? If it's you, then just quit giving her so much. There's a fine line between drinking to much fluids and not drinking enough, as you don't want her getting dehydrated, as that could cause far worse issues than wetting the bed.
Incontinence is very common with people who have any of the dementias, so it may not even be because of her drinking what you consider to be too much.
I would check with her urologist as they can prescribe some bladder control medications if needed, and definitely make sure that she's now wearing Depends(or something like them)and put down some water proof pads under her in the bed, so it won't soak through to the sheets and mattress.
I wish you the very best.

I agree with Countrymouse that you should treat this like a symptom of something else until it's proven it is just a "behavior" issue.

If you're having trouble getting her to adapt to incontinence briefs, remove all her underwear so that she only has the disposables to wear. Don't give her the option. Tell her some therapeutic fib as to where her "normal" undies disappeared to (the washer is broken, the washer ruined them, whatever).

Is this like a reflex habit, the drinking constantly, or could there be causes of actual thirst (such as undiagnosed diabetes, just as an example) or dry throat* (e.g. some medications)?

What is she drinking, and how much of it?

Is she wearing any continence products, such as pull-ups or wraparounds?

If you haven't already, get some washable waterproof mattress protectors and a bulk package of disposable bed pads. They won't solve the bed-wetting problem but they will make it less disastrous while you figure out a better way forward.

*One client who is on 24/7 oxygen keeps a glass of ice chunks by her all the time, and sucks on one when her throat feels rough.