perf cs-etm: Modularize auxtrace_buffer fetch function

perf cs-etm: Modularize auxtrace_buffer fetch function

Making the auxtrace_buffer fetch function modular so that it can be
called from different decoding context (timeless vs. non-timeless),
avoiding to repeat code.

No change in functionality is introduced by this patch.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Suzuki K Poulouse <suzuki.poulose@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/20190212171618.25355-14-mathieu.poirier@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>