drm/amd/amdgpu: validate the shadow BO.

Fixes a rare NULL pointer dereference in amdgpu_ttm_bind.

The issue was found by Nicolai Haehnle.
The patch was tested by Nicolai Haehnle.
Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

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

@@ -451,7 +451,7 @@ static int amdgpu_cs_validate(void *param, struct amdgpu_bo *bo)
 		return r;
 
 	if (bo->shadow)
-		r = amdgpu_cs_bo_validate(p, bo);
+		r = amdgpu_cs_bo_validate(p, bo->shadow);
 
 	return r;
 }