perf: Prevent waiting for cpu to come out of idle during read

Currently if the CPU is in idle state, the perf_event_read() loops
until it's out of that state. However, the PMU driver updates the
event->count value before going into idle. Hence, the caller of
perf_event_read() can simply return without waiting for the CPU to
come out of idle state and still get an updated value.

Change-Id: If0eb2f9bc64b5b0f2919e37d2c9b4dca5703ba46
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>