mem-cache: Fix fix of replacement count

mem-cache: Fix fix of replacement count

Commit 7976b561de61b7523ca9a860154ad7ba701d12a7 tried fixing
replacement update when a single location can be associated to
multiple blocks.

Although the comment of the correct action was added, the proper
validation check was forgotten. This change adds that check and
moves doing the eviction to when there is a valid block.

Change-Id: I31d8bb914ccfd1849e9d97464d70a58a62f59533
Signed-off-by: Daniel <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18210
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>