r300g: align vertex buffer suballocations to 4

src/gallium/drivers/r300/r300_context.c

     r300->context.create_video_buffer = vl_video_buffer_create;
 
     if (r300->screen->caps.has_tcl) {
-        r300->vbuf_mgr = u_vbuf_create(&r300->context, 1024 * 1024, 16,
+        r300->vbuf_mgr = u_vbuf_create(&r300->context, 1024 * 1024, 4,
                                        PIPE_BIND_VERTEX_BUFFER |
                                        PIPE_BIND_INDEX_BUFFER,
                                        U_VERTEX_FETCH_DWORD_ALIGNED);