mem: restructure Packet cmd initialization a bit more

mem: restructure Packet cmd initialization a bit more

Refactor the way that specific MemCmd values are generated for packets.
The new approach is a little more elegant in that we assign the right
value up front, and it's also more amenable to non-heap-allocated
Packet objects.

Also replaced the code in the Minor model that was still doing it the
ad-hoc way.

This is basically a refinement of http://repo.gem5.org/gem5/rev/711eb0e64249.