PM: EM: Add em_perf_state_from_pd() to get performance states table

Introduce a wrapper to get the performance states table of the performance
domain. The function should be called within the RCU read critical
section.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>