drm/i915/gt: Don't declare hangs if engine is stalled

If the ring submission is stalled on an external request, nothing can be
submitted, not even the heartbeat in the kernel context. Since nothing
is running, resetting the engine/device does not unblock the system and
is pointless. We can see if the heartbeat is supposed to be running
before declaring foul.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200528074109.28235-2-chris@chris-wilson.co.uk