perf evsel: Check if callchain is enabled before setting it up

The construct:

	if (callchain_param)
		perf_evsel__config_callchain(evsel, opts, &callchain_param);

happens in several places, so make perf_evsel__config_callchain() work
just like free(NULL), do nothing if param->enabled is not set.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Hendrick Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Richter <tmricht@linux.vnet.ibm.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-ykk0qzxnxwx3o611ctjnmxav@git.kernel.org


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