iris: Enable threaded shader compilation

There are a couple minor things that can be improved:

1. Eliminate (or reduce) the dynamic allocation of the
threaded_compile_job.

2. For apps like shader-db, improve the case where nr_threads=0.  Right
now this adds thread switching and mutex overhead.

3. Other performance improvements?  iris_uncompiled_shader::variants has
some special properties that make it ripe for replacement with a
lockless list.  Without gathering some data, it's hard to guess what
impact that could have.

v2: Fix whitespace and formatting issues.  Noticed by Ken.
s/threaded_compile_job/iris_threaded_compile_job/g.  Suggested by Ken.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11229>

src/gallium/drivers/iris/iris_screen.h

@@ -220,6 +220,8 @@ struct iris_screen {
    struct iris_bo *workaround_bo;
    struct iris_address workaround_address;
 
+   struct util_queue shader_compiler_queue;
+
    struct disk_cache *disk_cache;
 
    struct intel_measure_device measure;