perf evlist: Factor out a function to propagate maps for a single evsel

Subsequent fixes will need a function that just propagates maps for a
single evsel so factor it out.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Link: http://lkml.kernel.org/r/1441699142-18905-11-git-send-email-adrian.hunter@intel.com
[ Moved them to before perf_evlist__add() to avoid having to move it in the next patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>