perf evlist: No need to do any affinity setup when profiling pids

The cpumap is dummy, so no need to go on figuring out affinity.o

This way we reduce the setup time for simple scenarios like:

	$ perf stat sleep 1

Acked-by: Andi Kleen <andi@firstfloor.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>