perf hists: Clear dummy entry accumulated period

The accumulated period for dummy entry should also be 0.  Otherwise, the
total overhead could be overcounted.

  $ perf record -e '{LLC-load-misses,cpu/instructions/}' --call-graph=lbr ./tchain
  $ perf report --stdio
  # To display the perf.data header info, please use --header/--header-only options.
  #
  # Total Lost Samples: 0
  #
  # Samples: 21K of event 'anon group { LLC-load-misses, cpu/instructions/ }'
  # Event count (approx.): 16313667937
  #
  #         Children              Self  Command      Shared Object     Symbol
  # ................  ................  ...........  ................  ............................
  #
    4769.98%   0.01%     0.00%   0.01%  tchain_edit  [kernel.vmlinux]  [k] update_fast_timekeeper
    4356.18%   0.01%     0.00%   0.01%  tchain_edit  [kernel.vmlinux]  [k] trigger_load_balance
    3181.12%   0.01%     0.00%   0.01%  tchain_edit  [kernel.vmlinux]  [k] irq_work_tick
    1592.37%   0.00%     0.00%   0.00%  tchain_edit  [kernel.vmlinux]  [k] cpu_needs_another_gp

Signed-off-by: Kan Liang <kan.liang@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1461565689-5862-1-git-send-email-kan.liang@intel.com


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