intel/compiler: Pass backend_shader * to cfg_t()

As you can see, not having a pointer to the backend_shader from within
the class makes for some weird looking code.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4093>

src/intel/compiler/test_fs_scoreboard.cpp

@@ -73,14 +73,14 @@ lower_scoreboard(fs_visitor *v)
 
    if (print) {
       fprintf(stderr, "= Before =\n");
-      v->cfg->dump(v);
+      v->cfg->dump();
    }
 
    v->lower_scoreboard();
 
    if (print) {
       fprintf(stderr, "\n= After =\n");
-      v->cfg->dump(v);
+      v->cfg->dump();
    }
 }