drm/i915: Prefer checking the wakeref itself rather than the counter

The counter goes to zero at the start of the parking cycle, but the
wakeref itself is held until the end. Likewise, the counter becomes one
at the end of the unparking, but the wakeref is taken first. If we check
the wakeref instead of the counter, we include the unpark/unparking time
as intel_wakeref_is_active(), and do not spuriously declare inactive if
we fail to park (i.e. the parking and wakeref drop is postponed).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190503115225.30831-2-chris@chris-wilson.co.uk