perf top: Tell the user how to unfreeze events after pressing 'f'

When the user presses 'f' to disable events the visual cues are, well,
the percentages not changing and the number of events freezing.

Be more explicit by changing the help line at the bottom of the screen
to show the following messages when 'f' is pressed:

  "Press 'f' again to re-enable the events"

And then, when 'f' is pressed again:

  "Press 'f' to disable the events or 'h'
Suggested-by: NIngo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-uhiswg9a9rxm5gxg7ptjskjn@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-top.c

@@ -599,7 +599,13 @@ static void *display_thread_tui(void *arg)
 		 * No need to refresh, resort/decay histogram entries
 		 * if we are not collecting samples:
 		 */
-		hbt.refresh = top->evlist->enabled ? top->delay_secs : 0;
+		if (top->evlist->enabled) {
+			hbt.refresh = top->delay_secs;
+			help = "Press 'f' to disable the events or 'h' to see other hotkeys";
+		} else {
+			help = "Press 'f' again to re-enable the events";
+			hbt.refresh = 0;
+		}
 	}
 
 	done = 1;