Here is the thing. Time in the dream world goes quicker. So if the whole movie was a dream, wouldn't the characters be really old? The whole sequence of them planning the hiest spanned a couple months if I remembered right. Meaning if it was a dream, that would've been lots of decades. Making it impossible for them to be dreaming since none of them aged.
Time goes slower in a dream. As in you sleep for 1 hour but someone can be in the dream for 10 hours.For example the van was falling normal but in the next few levels it was like slow motion for them.