perf tools: Introduce callgraph_set for callgraph option

Introduce callgraph_set to indicate whether the callgraph option was set
by user.

Signed-off-by: Kan Liang <kan.liang@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1438162936-59698-4-git-send-email-kan.liang@intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>