perf stat: Factor out callback for collecting event values

To be used in next patch to support automatic summing of alias events.

v2: Move check for bad results to next patch
v3: Remove trivial addition.
v4: Use perf_evsel__cpus instead of evsel->cpus

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170320201711.14142-2-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>