drm/i915: i915_gem_object_sync must handle NULL

When I extracted the synchronization code for implementing semaphorified
pageflips (74f5f6e0), I neglected the non pipelined case which also
calls this code. The modesetting code wants to make sure the object has
finished rendering to the frame before configuring the scanout (ie.
non-pipelined case).

As a result of a follow on discussion on IRC, I've decided to add a
comment about the function itself which received much inspiration from
Chris as well. So really, this patch was ghost-written by Chris :).
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -1953,6 +1953,18 @@ i915_gem_object_wait_rendering(struct drm_i915_gem_object *obj)
 	return 0;
 }
 
+/**
+ * i915_gem_object_sync - sync an object to a ring.
+ *
+ * @obj: object which may be in use on another ring.
+ * @to: ring we wish to use the object on. May be NULL.
+ *
+ * This code is meant to abstract object synchronization with the GPU.
+ * Calling with NULL implies synchronizing the object with the CPU
+ * rather than a particular GPU ring.
+ *
+ * Returns 0 if successful, else propagates up the lower layer error.
+ */
 int
 i915_gem_object_sync(struct drm_i915_gem_object *obj,
 		     struct intel_ring_buffer *to)
@@ -1964,7 +1976,7 @@ i915_gem_object_sync(struct drm_i915_gem_object *obj,
 	if (from == NULL || to == from)
 		return 0;
 
-	if (!i915_semaphore_is_enabled(obj->base.dev))
+	if (to == NULL || !i915_semaphore_is_enabled(obj->base.dev))
 		return i915_gem_object_wait_rendering(obj);
 
 	idx = intel_ring_sync_index(from, to);