mem-cache: Fix recvTimingReq doWritebacks tick

mem-cache: Fix recvTimingReq doWritebacks tick

Before being sent to the writebuffer, the evicted blocks
must be selected for replacement, and therefore the
access latency must be applied. The forward latency is
then applied on top of that delay.

Change-Id: I16a25a8bf6051f63eb7a02fe66acb6af26d434fc
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/14736
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>