perf, x86: implement group scheduling transactional APIs

Convert to the transactional PMU API and remove the duplication of
group_sched_in().

Reviewed-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: David Miller <davem@davemloft.net>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1272002172.5707.61.camel@minggr.sh.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>