mem-cache: Move unused prefetches counter update

mem-cache: Move unused prefetches counter update

The number of unused prefetches should be updated every time
a block is invalidated, therefore we move the update to within
the corresponding function.

Change-Id: If3ac2ea43611525bd3c36d628d88382042fcb7dc
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18908
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>