drm/i915: Wait for engines to idle before retiring

In the next^W forthcoming patch, we will start to defer retiring the
request from the engine list if it is still active on the submission
backend. To preserve the semantics that after wait-for-idle completes
the system is idle and fully retired, we need to therefore wait for the
backends to idle before calling i915_retire_requests().
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180627115334.16282-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -3810,10 +3810,13 @@ int i915_gem_wait_for_idle(struct drm_i915_private *i915, unsigned int flags)
 			if (err)
 				return err;
 		}
+
+		err = wait_for_engines(i915);
+		if (err)
+			return err;
+
 		i915_retire_requests(i915);
 		GEM_BUG_ON(i915->gt.active_requests);
-
-		return wait_for_engines(i915);
 	} else {
 		struct intel_engine_cs *engine;
 		enum intel_engine_id id;
@@ -3824,9 +3827,9 @@ int i915_gem_wait_for_idle(struct drm_i915_private *i915, unsigned int flags)
 			if (err)
 				return err;
 		}
-
-		return 0;
 	}
+
+	return 0;
 }
 
 static void __i915_gem_object_flush_for_display(struct drm_i915_gem_object *obj)