perf stat: Update POWER9 metrics to utilize other metrics

These changes take advantage of the new capability added in merge commit
00e4db51 "Allow using computed metrics
in calculating other metrics".

The net is a simplification of the expressions for a handful of metrics,
but no functional change.
Signed-off-by: Paul Clarke <pc@us.ibm.com>
Reviewed-by: Kajol Jain <kjain@linux.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: http://lore.kernel.org/lkml/20200813222155.268183-1-pc@us.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>