drm/amdgpu: simplify VM shadow handling v2

Now that we don't join PTE updates any more we don't need to call
the update function twice for this.

v2: rebased

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>