mem-cache: Populate whenReady for blocks filled from writebacks

mem-cache: Populate whenReady for blocks filled from writebacks

Writebacks write data to either an existing block or a newly allocated
block. In either case we need to populate the whenReady field of the
block which will determine when the new value can be used.

Change-Id: I5788fad0b8086a1be96714639bf6a9470b334926
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8285
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>