anv: Refactor layout_to_aux_usage

This makes the whole function far more obvious. You no longer have to reason about what image->planes[plane].aux_usage is and what happens if it happens to be NONE in the context of some aspect. Instead, it's straightforward what the function does for color, depth, and stencil.

This MR is broken out of !2605 (merged) due to ongoing discussions on !2605 (comment 382040) I'll try to copy the discussion below.