on the one hand, i tend to doubt Apple would jump into a chip market that's already already serving them very well. they make chips for mobile because no one's creating what they need. but for laptop/desktop cpu and gpu, they're very well served.
on the other hand, apple loves to surprise people.
but if it were to happen, it would be quite a while off. it's very different beast making a GPU for a computer than for a cell phone or tablet