drm/amdgpu: Revert "kmap PDs/PTs in amdgpu_vm_update_directories"

This reverts commit a7f91061.

Felix pointed out that we need to have the BOs mapped even before
amdgpu_vm_update_directories is called.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>