x86, perf: Allow zero PEBS status with only single active event

Normally we drop PEBS events with a zero status field. But when
there is only a single PEBS event active we can assume the
PEBS record is for that event. The PEBS buffer is always flushed
when PEBS events are disabled, so there is no risk of mishandling
state PEBS records this way.

Signed-off-by: Andi Kleen <ak@linux.intel.com>