drm/i915: Convert intel_rc6_residency_us to ns

Will be used for exposing the PMU counters.

v2:
 * Move intel_runtime_pm_get/put to the callers. (Chris Wilson)
 * Restore full unit conversion precision.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171121181852.16128-8-tvrtko.ursulin@linux.intel.com