drm/amdgpu: stop reserving a shared fence for VA updates

We don't add any fences do the buffer, but just use it's address.

Additional to that we don't need a duplicates list here.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>