drm/i915: Move on the new pm runtime interface

Use the new PM-runtime interface to get the accounted suspended time:
pm_runtime_suspended_time().

This new interface helps to simplify and cleanup the code that computes
__I915_SAMPLE_RC6_ESTIMATED and to remove direct access to internals of
PM-runtime.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>