The question is rather What is "reality": the dream (et al.) or the physical world (what you describe as reality). See Descartes first two meditations (and note that he relies fully on the existence of God to prove the existence of reality later). In this case, us experiencing a "dream" just serves to outline the point; Descartes, for example, also suggests that we are being fooled by an evil daemon. If it's a dream or an evil daemon — doesn't matter; it would likely be something entirely beyond our comprehension anyway. But genuinely proving the physical world as being reality is very difficult.