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_context.h

@@ -412,6 +412,9 @@ struct iris_uncompiled_shader {
 
    /** Lock for the variants list */
    simple_mtx_t lock;
+
+   /** For parallel shader compiles */
+   struct util_queue_fence ready;
 };
 
 enum iris_surface_group {