mem-ruby: Fix missing TBE allocation and deallocation

The TBE allocation and deallcation are currently missing during
the directory state transition from I to M in protocol MI_example.

Change-Id: If7569c02faf56ea84c34ee1345f1a33d318cdfff
Signed-off-by: Zicong Wang <wangzicong@nudt.edu.cn>
Reviewed-on: https://gem5-review.googlesource.com/c/15535
Reviewed-by: Pouya Fotouhi <pfotouhi@ucdavis.edu>
Maintainer: Jason Lowe-Power <jason@lowepower.com>