asahi: Linux porting, part 1

@asahilina has been working on a branch that integrates Mesa with her new UAPI to run the Asahi driver on Linux. Today's project was cleaning up that branch as I took the shiny new driver for a spin. https://gitlab.freedesktop.org/alyssa/mesa/-/tree/asahi/rebased contains the fruit of that effort. Unfortunately, we can't merge Linux UAPI code until the corresponding code is merged into the upstream Linux kernel, which could take a long time. In the interest of reducing rebase pain in the mean time, this series contains the non-UAPI bits, some of which will be immediately useful.