perf: Improve idle handling for perf events

Use the exclude_idle attribute of the perf events to avoid reading
PMUs of idle CPUs. The counter values are updated when CPU enters
idle and the saved value is returned when the idle CPU is queried
for that event provided the attribute is set in the perf_event.

Change-Id: Ia4f7b8106afdf53ee6f31049b5ef799afbda8480
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>