drm/i915/gt: Move the heartbeat into the high priority system wq

As we ensure that the heartbeat is reasonably fast (and should not
block), move the heartbeat work into the system_highpri_wq to avoid
having this essential task be blocked behind other slow work, such as
our own retire_work_handler.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/2119
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/20200702095219.963-2-chris@chris-wilson.co.uk