It has nothing to do with being transgender, but the fact that a girl will be assigned a male roommate, and does have the right, probably given to her by university rules, to a roommate of the same gender.
The problem here is that the OP was forced by his parents to choose housing based on his sex at birth.
I think overall people won’t mind and many will be supportive. I have a friend in your exact situation but is now a sophomore, and he didn’t have any big dorm troubles that I heard of. His school was known to be pretty progressive though (Brown).
I would tell your roommate ASAP though, as uncomfortable as it may be. It will be better for you as well, as it gives you an opportunity to switch if she would not be accepting of you.