perf c2c: Fix display bug when using pipe

Currently 'perf c2c report' determines display mode using the --stdio
option, but it could be a problem if stdout is not a tty since
setup_browser falls back to stdio in this case.

But perf c2c didn't know this and tried to use TUI browser anyway.  It
should check "use_browser" variable instead.

For example, the following command showed nothing and broke terminal
setting.  Now it's fixed..

  $ perf c2c report | head
  =================================================
              Trace Event Information
  =================================================
    Total records                     :        136
    Locked Load/Store Operations      :          6
    Load Operations                   :         62
    Loads - uncacheable               :          0
    Loads - IO                        :          1
    Loads - Miss                      :          7
    Loads - no mapping                :          2

Committer notes:

When trying it without a proper perf.data file it results in a stuck
terminal, just as Namhyung reported above:

  [acme@jouet ~]$ perf c2c report | head
  WARNING: no sample cpu value[acme@jouet ~]$

One has to kill it from some other xterm. Confirm that this patch fixes
it:

After:

  $ perf c2c report | head
  WARNING: no sample cpu value=================================================
              Trace Event Information
  =================================================
    Total records                     :         14
    Locked Load/Store Operations      :          0
    Load Operations                   :          0
    Loads - uncacheable               :          0
    Loads - IO                        :          0
    Loads - Miss                      :          0
    Loads - no mapping                :          0
  $

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kernel-team@lge.com
Link: http://lkml.kernel.org/r/20170307150851.22304-6-namhyung@kernel.org


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