perf ui browser: Allow specifying message to show when no samples are available to display

The 'perf top' tool will use that to avoid having a initial blank screen
while collecting the minimum number of samples to sort and display.

Cc: Adrian Hunter <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/ui/browser.c

···
 	SLsmg_fill_region(browser->y + row + browser->extra_title_lines, browser->x,
 			  browser->rows - row, width, ' ');
 
+	if (browser->nr_entries == 0 && browser->no_samples_msg)
+		__ui__info_window(NULL, browser->no_samples_msg, NULL);
 	return 0;
 }
 

tools/perf/ui/browser.h

···
 	void	      *priv;
 	const char    *title;
 	char	      *helpline;
+	const char    *no_samples_msg;
 	void 	      (*refresh_dimensions)(struct ui_browser *browser);
 	unsigned int  (*refresh)(struct ui_browser *browser);
 	void	      (*write)(struct ui_browser *browser, void *entry, int row);