drm/i915/gt: Cleanup heartbeat systole first

Before we grab the engine wakeref, tidy up the previous heartbeat
request. If we then abort because the engine powerwell is off, we ensure
the request is freed as we know we will not have freed it when
cancelling the work (as the work is running!).

Fixes: 841e8672 ("drm/i915/gt: Only drop heartbeat.systole if the sole owner")
References: 058179e7 ("drm/i915/gt: Replace hangcheck by heartbeats")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106223410.30334-1-chris@chris-wilson.co.uk