perf target: Move the checking of which map function to call into function.

Check for cpu_map__dummy_new() or cpu_map__new() to be called in
perf_evlist__create_maps() is more complicated.

This patch moves the checking work into target.h, combining two
conditions and making perf_evlist__create_maps() more readable.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/b8c41f1fd2c4f0df71eb7b19aea74fb64d46cdda.1386197481.git.yangds.fnst@cn.fujitsu.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>