perf intel-pt: Tidy old_buffer handling in intel_pt_get_trace()

perf intel-pt: Tidy old_buffer handling in intel_pt_get_trace()

intel_pt_get_trace() fixes overlaps between the current buffer and the
previous buffer ('old_buffer').

However the previous buffer might not have had usable data (no PSB) so
the comparison must be made against the previous buffer that had usable
data.

Tidy that by keeping a pointer for that purpose in struct intel_pt_queue.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1520431349-30689-8-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>