Nobody knows if the reality, the way we see it, actually exists. It's that old René Descartes thing where we can only be sure of our own existence: "Cogito ergo sum", I think, therefore I am. But, just because we can't be 100% sure, doesn't mean it isn't our reality. There's really no compelling reason to dismiss our own reality as fake, so it's just better to assume this is reality, and our dreams are just dreams.

P.S., Déjà vu is just a common psychological phenomena. Nothing special.