radeonsi: add another aux context for uploading shaders

radeonsi: add another aux context for uploading shaders

When the first auxiliary context is locked and wants to compile and upload
a shader asynchronously, we need to use another auxiliary context
in the compiler thread because the first one is locked at that point.

This adds an array of auxiliary contexts into si_screen and changes how aux
contexts are accessed.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25494>