radeonsi: fix potential use-after-free of debug callbacks

Found by inspection.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
diff --git a/src/gallium/drivers/radeonsi/si_pipe.c b/src/gallium/drivers/radeonsi/si_pipe.c
index 7940fc8..7c50ffa 100644
--- a/src/gallium/drivers/radeonsi/si_pipe.c
+++ b/src/gallium/drivers/radeonsi/si_pipe.c
@@ -158,6 +158,10 @@
 				  const struct pipe_debug_callback *cb)
 {
 	struct si_context *sctx = (struct si_context *)ctx;
+	struct si_screen *screen = sctx->screen;
+
+	util_queue_finish(&screen->shader_compiler_queue);
+	util_queue_finish(&screen->shader_compiler_queue_low_priority);
 
 	if (cb)
 		sctx->debug = *cb;