perf intel-pt: Synthesize CBR events when last seen value changes

The first core-to-bus ratio (CBR) event will not be shown if --itrace
's' option (skip initial number of events) is used, nor if time
intervals are specified that do not include the start of tracing. Change
the logic to record the last CBR value seen by the user, and synthesize
CBR events whenever that changes.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190622093248.581-5-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>