gallium: Fix mesa format name in unit test failure path.

We clearly wanted the mesa format here.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Acked-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/tests/st_format.c

@@ -92,7 +92,7 @@ int main(int argc, char **argv)
          if (pf != i) {
             fprintf(stderr, "Round-tripping %s -> %s -> %s failed\n",
                     util_format_short_name(i),
-                    _mesa_get_format_name(pf),
+                    _mesa_get_format_name(mf),
                     util_format_short_name(pf));
             return 1;
          }