perf: Optimize group_sched_in()

perf: Optimize group_sched_in()

Use the ctx pmu instead of the event pmu.

When a group leader is a software event but the group contains
hardware events, the entire group is on the hardware PMU.

Using the hardware PMU for the transaction makes most sense since
that's the most expensive one to programm (and software PMUs generally
don't have TXN support anyway).

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-sctoo9t2f3nn2c9g568928q3@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>