drm/amdgpu: clear ring pointer in amdgpu_device on teardown

This is in symmetry to setup done in amdgpu_ring_init.
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c

@@ -264,6 +264,8 @@ void amdgpu_ring_fini(struct amdgpu_ring *ring)
 			      (void **)&ring->ring);
 
 	amdgpu_debugfs_ring_fini(ring);
+
+	ring->adev->rings[ring->idx] = NULL;
 }
 
 /*