drm/amdgpu: fix PRT teardown on VM fini v3

v2: new approach fixing this by registering a fence callback for
    all users of the VM on teardown
v3: agd: rebase

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>