intel/perf: fix invalid hw_id in query results

Accumulation happens between 2 reports, it can be between a start/end
report from another context. So only consider updating the hw_id of
the results when it's not already valid and that we have a valid value
to put in there.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 41b54b5f ("i965: move OA accumulation code to intel/perf")
Reviewed-by: Mark Janes <mark.a.janes@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit acea59db)