drm/i915: Leave engine parking to the engines

Drop the check in GEM parking that the engines were already parked. The
intention here was that before we dropped the GT wakeref, we were sure
that no more interrupts could be raised -- however, we have already
dropped the wakeref by this point and the warning is no longer valid.

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/20190502150024.16636-2-chris@chris-wilson.co.uk