perf hists browser: Dynamically change verbosity level

Here is a small patch which tries to fulfill a point in the perf todo
list:

* Make pressing 'V' multiple times to go on cycling thru various
  verbosity levels in 'perf top', so that info that is present in
  'perf top -v' can be obtained without having to restart the tool
  (acme).

After a small grep in the code, the max verbosity level seems 3; so,
we cycle at 4; I did not dare define a MAX_VERBOSE_LEVEL constant.

Signed-off-by: Alexis Berlemont <alexis.berlemont@gmail.com>
Suggested-and-Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20161012214823.14324-2-alexis.berlemont@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>