drm/i915/execbuffer: On error, starting unwinding from the previous object

As the error occurred on the current object, it means that its state was
not changed and so it should be excluded from the unwind.
Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -530,6 +530,9 @@ i915_gem_execbuffer_reserve(struct drm_device *dev,
 	} while (1);
 
 err:
+	obj = list_entry(obj->exec_list.prev,
+			 struct drm_i915_gem_object,
+			 exec_list);
 	while (objects != &obj->exec_list) {
 		if (obj->gtt_space)
 			i915_gem_object_unpin(obj);