drm/i915: Update ring position from request on retiring

When wedged, we do not update the ring->tail as we submit the requests
causing us to leak the ring->space upon cleaning up the wedged driver.
We can just use the value stored in rq->tail, and keep the submission
backend details away from set-wedge.
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/20180307134226.25492-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_request.c

@@ -358,7 +358,7 @@ static void advance_ring(struct i915_request *request)
 		 * is just about to be. Either works, if we miss the last two
 		 * noops - they are safe to be replayed on a reset.
 		 */
-		tail = READ_ONCE(request->ring->tail);
+		tail = READ_ONCE(request->tail);
 	} else {
 		tail = request->postfix;
 	}

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1593,6 +1593,7 @@ static noinline int wait_for_space(struct intel_ring *ring, unsigned int bytes)
 	if (intel_ring_update_space(ring) >= bytes)
 		return 0;
 
+	GEM_BUG_ON(list_empty(&ring->request_list));
 	list_for_each_entry(target, &ring->request_list, ring_link) {
 		/* Would completion of this request free enough space? */
 		if (bytes <= __intel_ring_space(target->postfix,