perf top: Lift restriction on using callchains without "sym" in --sort

This restriction is not present in 'perf report' and since 'perf top'
uses the same hists browser, remove it from it as well.

With this we create per event buckets with callchain trees, so that

  # perf top --sort dso -g --no-children

Bucketizes samples by DSO and below it shows the callchains leading to
functions in this DSO.

Try also:

  # perf top -e sched:*switch -g --no-children

To see the callchains leading to sched switches, pressing 'E' to expand
all one can quickly see the most common scheduler switches and what
leads to them, for instance, calls to IO, futexes, etc.

Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lkml.kernel.org/r/20190107140854.GA28965@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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