freedreno/ir3: Reduce compiler thread pool size

With the current scheme, looking at game startup which should be the
worst case (most heavily loaded) time for the compiler threads, and they
seem to be ~10% busy.  Furthermore we typically have a mix of "big" and
"LITTLE" cores.. with about half being "big".  So sizing the thread pool
to the half the # of CPU cores seems reasonable.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19387>

src/gallium/drivers/freedreno/ir3/ir3_gallium.c

@@ -556,7 +556,7 @@ ir3_screen_init(struct pipe_screen *pscreen)
     * big cores.  OTOH if they are sitting idle, maybe it is useful to
     * use them?
     */
-   unsigned num_threads = sysconf(_SC_NPROCESSORS_ONLN) - 1;
+   unsigned num_threads = sysconf(_SC_NPROCESSORS_ONLN) / 2;
 
    /* Create at least one thread - even on single core CPU systems. */
    num_threads = MAX2(1, num_threads);