[Nouveau] NVA3 clock tree improvements

Following a series of patches to improve nouveaus clock tree parsing. Reclocking these engines (all but memory) is pretty stable on the cards I've tested. Please review and merge when approved. These patches do not solve the problem that core/shader engine doesn't like to be clocked up too far without fb following, with visible corruption as a result. I suspect this problem is unrelated and requires a separate patch towards context creation to even things out. Roy