st/mesa: notify u_vbuf/driver that draw index bounds are unknown for indirect

Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/state_tracker/st_draw.c

@@ -261,6 +261,7 @@ st_indirect_draw_vbo(struct gl_context *ctx,
    memset(&indirect, 0, sizeof(indirect));
    util_draw_init_info(&info);
    info.start = 0; /* index offset / index size */
+   info.max_index = ~0u; /* so that u_vbuf can tell that it's unknown */
 
    if (ib) {
       struct gl_buffer_object *bufobj = ib->obj;