mem: Fix latency handling in MemDelay

mem: Fix latency handling in MemDelay

MemDelay wouldn't consume pre-existing delays in the packet and
therefore the latency it adds would overlap with them. This patch
fixes the MemDelay to properly account for them.

Change-Id: I7330fbf1c8161a21523a0b4aab31c72e34bce650
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30055
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>