drm/amdgpu: Revert "add spin lock to protect freed list in vm (v3)"

Not needed any more because we need to protect the elements on the list anyway.

This reverts commit dae6ecf9e6c9b677e577826c3ac665c6dd9c490b.

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