drm/amdgpu: Revert "add lock for interval tree in vm"

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

This reverts commit fe237ed7efec8ac147a4572fdf81173a7f8ddda7.

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>