cpu-o3: fix data pkt initialization for split load

cpu-o3: fix data pkt initialization for split load

When a split load hits a memory region where IPRs are mapped, the
Writebackevent which is scheduled for that was carrying a data packet
that was not correctly initialized which caused an assertion to fire
when the Writeback event is processed.

Change-Id: I71a4e291f0086f7468d7e8124a0a8f098088972f
Signed-off-by: Matthias Hille <matthiashille8@gmail.com>
Reported-by: Matthias Hille <matthiashille8@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/4620
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>