perf ordered_events: Add first_time() method

To get the timestamp in the first event in the queue.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Dmitry Levin <ldv@altlinux.org>
Cc: Eugene Syromiatnikov <esyr@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Luis Cláudio Gonçalves <lclaudio@uudg.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/n/tip-appp27jw1ul8kgg872j43r5o@git.kernel.org
[ split from a larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>