perf cpumap: Add is_dummy() method

Needed to check if a cpu_map is dummy, i.e. not a cpu map at all, for
pid monitoring scenarios.

This probably needs to move to libperf, but since perf itself is the
first and so far only user, leave it at tools/perf/util/.

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>