radeonsi: always use async compiles when creating shader/compute states

radeonsi: always use async compiles when creating shader/compute states

With Gallium threaded contexts, creating shader/compute states is
effectively a screen operation, so we should not use context state.

In particular, this allows us to avoid using the context's LLVM
TargetMachine.

This isn't an issue yet because u_threaded_context filters out non-async
debug callbacks, and we disable threaded contexts for debug contexts.
However, we may want to change that in the future.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>