drm/i915: Fix incorrect assert about pending power domain async-put work

It's possible that an already dequeued put_async_work() will release the
reference (*) that was put asynchronously after the dequeue happened.
This leaves an async-put work pending, without any reference to release.
A subsequent async-put may trigger the

drm_WARN_ON(!queue_delayed_work(&power_domains->async_put_work));

warn due to async_put_work() still pending. To avoid the warn, cancel
the pending async_put_work() when releasing the reference at (*) above.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/3421
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2289


Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210526143729.2563672-2-imre.deak@intel.com