panfrost: Refactor attributes and varyings to use GenXML

Now that the initial GenXML proof-of-concept is merged and landed, we can start porting over bigger structures. This series focuses on attributes and attribute buffers (and equivalently varyings), porting over to use the new infrastructure for better ergonomics, performance, and stability. Indeed - the change set nets 372 lines deleted, two accidental CPU reads of GPU buffers were identified and fixed, and the remaining draw.random.* failures are fixed. Overall, I'm quite happy and cautiously optimistic about the next structures :)