drm/i915/pmu: Use GT parked for estimating RC6 while asleep

As we track when we put the GT device to sleep upon idling, we can use
that callback to sample the current rc6 counters and record the
timestamp for estimating samples after that point while asleep.

v2: Stick to using ktime_t
v3: Track user_wakerefs that interfere with the new
intel_gt_pm_wait_for_idle
v4: No need for parked/unparked estimation if !CONFIG_PM
v5: Keep timer park/unpark logic as was
v6: Refactor duplicated estimate/update rc6 logic
v7: Pull intel_get_pm_get_if_awake() out from the pmu->lock.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105010
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/20190912124813.19225-1-chris@chris-wilson.co.uk