The only quotes I've seen from Cameron on the subject say that he'd had the idea for Avatar before Titanic, which came out in 1997. Production on Titanic started in 1995, and pre-production started even earlier than that. A very close friend of mine did some very early R&D work on CG musculature for Cameron back in 1994. That work was too complex for the digital characters used for Titanic, and, IIRC, was driven by the desire to find out if the technology was capable of producing digital characters that could be the main characters of a film. And oh, look, what do you know, he wrote his script in 1994, which was before Pocahontas came out.

Not to mention, the story of Pocahontas has been around since the early 1600's. It's not like Cameron would have needed to steal it from Disney. (Now there's a company that's absolutely shameless about lifting material.)