drm/amdgpu: kmap PDs/PTs in amdgpu_vm_update_directories

In theory it is possible that PDs/PTs can move without eviction.

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