It's clear that Chris Rock is talking about black people and white people. I think the idea is that white people are told what they can do, whereas black people are told what they can't do. It's the same phrase in essence, however the focus (the first thing to look at) for white people is "the sky". The focus for black people is "the limit".

I suppose the question should be "When you're white, the sky' s the limit; when you're black, the limit' the sky?" Since I have not watched that movie, I don't know what "white" and "black" are referred to under that particular circumstance. But I think "the sky's the limit" means nothing is impossible and one can chase dreams, seize the opportunities and his fate is in his own hand while "the limit's the sky" means the opposite situation.