drm/i915/gt: Only drop heartbeat.systole if the sole owner

Mika spotted that only using cancel_delayed_work() could mean that we
attempted to clear the heartbeat.systole while the worker was still
running. Rectify the situation by only touching the systole from outside
the worker if we suceeded in cancelling the worker before it could run.
The worker is expected to clean up by itself upon idling.

Reported-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Fixes: 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/20191106133129.17732-1-chris@chris-wilson.co.uk