Wasn't it preordained that Joseph would have to go down to Egypt. Why then does rashi attribute yosef's descent to Egypt based on the fact that he wished to live in peace and tranquility?

Yes it was preordained but not that he would not be allowef to know of his whereabouts. By wishing to live in peace and tranquility this added to not only Joseph descending to Egypt but also that yakov would not know of his whereabouts for 22 years.