I have not stayed in a resort during Christmas, but based on local info I would say this is not too early to book. WDW is extremely popular the weeks up to and including Christmas. Crowds are there, but with the exception of Christmas Day, nothing like what you would see in the summer. And the weather is MUCH nicer.
I am a FL resident and annual passholder. I have never seen discounts that include Christmas. If you are willing to go a week or so before the holiday, you might find some but they would not be advertised yet. It is a tough call - booking now and guaranteeing your space or waiting for a possible discount. You could call and ask the CM if a discount becomes available later if you could take advantage of it - even if you already booked.
HTH!
Carolyn