drm/amdgpu: sync the VM PD/PT before clearing it

Otherwise we might overwrite stuff which is still in use.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>