perf stat: Factor out prefix display

The prefix is needed for interval mode to print timestamp at the
beginning of each line.  But the it's tricky for the metric only
mode since it doesn't print every evsel and combines the metrics
into a single line.

So it needed to pass 'first' argument to print_counter_aggrdata()
to determine if the current event is being printed at first.  This
makes the code hard to read.

Let's move the logic out of the function and do it in the outer
print loop.  This would enable further cleanups later.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20221114230227.1255976-17-namhyung@kernel.org


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