asahi: Augment PBE descriptor for software access

asahi: Augment PBE descriptor for software access

For implementing image atomics (and multisample image writes), we need
information about the image layout in the shader. It's a lot nicer to determine
the image layouts on the CPU (where we have ail) and stash the results in the
PBE descriptor, where we have a convenient hole to do so, rather than trying to
do all the layout calculations on the GPU on the fly. Add a data structure that
the driver will fill out and the image atomic lowering will consider as part of
the hardware.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24258>