drm/i915/pmu: Add RC6 residency metrics

For clients like intel-gpu-overlay it is easier to read the
counters via the perf API than having to parse sysfs.

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/20171121181852.16128-9-tvrtko.ursulin@linux.intel.com