perf report: Add support to display group output for non group events

Add support to display group output for if non grouped events are
detected and user forces --group option. Now for non-group events
recorded like:

  $ perf record -e 'cycles,instructions' ls

you can still get group output by using --group option
in report:

  $ perf report --group --stdio
  ...
  #         Overhead  Command  Shared Object     Symbol
  # ................  .......  ................  ......................
  #
      17.67%   0.00%  ls       libc-2.25.so      [.] _IO_do_write@@GLIB
      15.59%  25.94%  ls       ls                [.] calculate_columns
      15.41%  31.35%  ls       libc-2.25.so      [.] __strcoll_l
  ...

Committer note:

We should improve on this by making sure that the first line states that
this is not a group, but since the user doesn't have to force group view
when really using grouped events (e.g. '{cycles,instructions}'), the
user better know what is being done...

Requested-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Stephane Eranian <eranian@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180209092734.GB20449@krava


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