drm/amdgpu: install stub fence into potential unused fence pointers
[ Upstream commit 187916e6 ] When using cpu to update page tables, vm update fences are unused. Install stub fence into these fence pointers instead of NULL to avoid NULL dereference when calling dma_fence_wait() on them. Suggested-by: Christian König <christian.koenig@amd.com> Signed-off-by: Lang Yu <Lang.Yu@amd.com> Reviewed-by: Christian König <christian.koenig@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com> Signed-off-by: Sasha Levin <sashal@kernel.org>