mem: Align the snoop behavior in the XBar for atomic and timing

mem: Align the snoop behavior in the XBar for atomic and timing

When the XBar receives a Writeback/WriteClean packet, it doesn't need
to snoop the upstream caches. It only queries the snoop filter and
sets the blockCached flag accordingly. This is in line with the
recvTimingReq.

Change-Id: I0ae22f21491d75a111019124bb95bac7b16d3cd3
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Anouk Van Laer <anouk.vanlaer@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5043
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>