iris: destroy our mutexes a little later

While there seems to be no bug with the state things are today, I was recently doing some debugging and put an iris_bo_wait() before a bo_close() in iris_bufmgr_destroy(), which caused an issue since the bo_deps_lock mutex had already been destroyed.

Since there are quite a few things we do with the bufmgr after destroying the mutexes, I figured we should probably postpone mutex destruction in order to be a little safer against future code modifications like the one I just did.