drm/i915/gt: Replace open-coded intel_engine_stop_cs()

In the legacy ringbuffer submission, we still had an open-coded version
of intel_engine_stop_cs() with one additional verification step. Transfer
that verification to intel_engine_stop_cs() itself, and call it.

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/20210113204709.15020-1-chris@chris-wilson.co.uk