vk/formats: Document new meaning of anv_format::cpp

vk/formats: Document new meaning of anv_format::cpp

The way the code currently works is that anv_format::cpp is the cpp of
anv_format::surface_format.

Me and Kristian disagree about how the code *should* work. Despite that,
I think it's in our discussion's best interest to document how the code
*currently* works. That should eliminate confusion.

If and when the code begins to work differently, then we'll update the
anv_format comments.