mesa/vbo: add some missing fallthrough comments

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5705>

src/mesa/vbo/vbo_save_api.c

@@ -739,10 +739,13 @@ copy_from_current(struct gl_context *ctx)
       switch (save->attrsz[i]) {
       case 4:
          save->attrptr[i][3] = save->current[i][3];
+         /* fallthrough */
       case 3:
          save->attrptr[i][2] = save->current[i][2];
+         /* fallthrough */
       case 2:
          save->attrptr[i][1] = save->current[i][1];
+         /* fallthrough */
       case 1:
          save->attrptr[i][0] = save->current[i][0];
          break;