perf/x86/intel/bts: Set event->hw.itrace_started in pmu::start to match the new logic

perf/x86/intel/bts: Set event->hw.itrace_started in pmu::start to match the new logic

Since event->hw.itrace_started is now set in pmu::start() to signal the beginning of
the trace, do so also in the intel_bts driver.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: acme@infradead.org
Cc: adrian.hunter@intel.com
Cc: hpa@zytor.com
Link: http://lkml.kernel.org/r/1437140050-23363-4-git-send-email-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>