drm/i915/gt: Clear rc6 residency trackers across suspend

drm/i915/gt: Clear rc6 residency trackers across suspend

On suspend, the rc6 residency counters (stored in HW registers) will be
lost and cleared. However, we keep track of the rc6 residency to provide
a continuous 64b sampling, and if we see the HW value go backwards, we
assume it overflowed and add on 32b/40b -- an interesting artifact when
sampling across suspend.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200114105648.2172026-2-chris@chris-wilson.co.uk