mem-cache: Move reference count stats update to blk invalidation

mem-cache: Move reference count stats update to blk invalidation

The tags in the cache keep track of the number of references to the
blocks as well as the average number of references between an
insertion and the next invalidation. Previously the stats where
updated only on block insertion and invalidations were ignored. This
changes moves the update of the counters to the block invalidation
function.

Change-Id: Ie7672c13813ec278a65232694024d2e5e17c4612
Reviewed-on: https://gem5-review.googlesource.com/10428
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>