cpuidle: play_idle: Specify play_idle with an idle state

Currently, the play_idle function does not allow to tell which idle
state we want to go. Improve this by passing the idle state as
parameter to the function.

Export cpuidle_find_deepest_state() symbol as it is used from the
intel_powerclamp driver as a module.

There is no functional changes, the cpuidle state is the deepest one.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Mathieu Poirier <mathieu.poirier@linaro.org>