asahi: rename Null layout

this isn't a Null layout only, it's also used with sparse. update the name to
reduce confusion. Unsure if we have a use case yet but maybe as an optimization
later?

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33682>
2 files changed