drm/nouveau/graph/nvc0: Fix engine pointer retrieval

Other methods in this file suggest this is the correct way to retrieve
the engine pointer.

Signed-off-by: Lauri Peltonen <lpeltonen@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>