perf hists: Check if a hist_entry has callchains before using them

So far if we use 'perf record -g' this will make
symbol_conf.use_callchain 'true' and logic will assume that all events
have callchains enabled, but ever since we added the possibility of
setting up callchains for some events (e.g.: -e
cycles/call-graph=dwarf/) while not for others, we limit usage scenarios
by looking at that symbol_conf.use_callchain global boolean, we better
look at each event attributes.

On the road to that we need to look if a hist_entry has callchains, that
is, to go from hist_entry->hists to the evsel that contains it, to then
look at evsel->sample_type for PERF_SAMPLE_CALLCHAIN.

The next step is to add a symbol_conf.ignore_callchains global, to use
in the places where what we really want to know is if callchains should
be ignored, even if present.

Then -g will mean just to select a callchain mode to be applied to all
events not explicitely setting some other callchain mode, i.e. a default
callchain mode, and --no-call-graph will set
symbol_conf.ignore_callchains with that clear intention.

That too will at some point become a per evsel thing, that tools can set
for all or just a few of its evsels.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-0sas5cm4dsw2obn75g7ruz69@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>