drm/amdgpu: use a sync object for VMID fences v2

v2: rebase & cleanup

This way we can store more than one fence as user for each VMID.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Reviewed-by: Chunming Zhou <david1.zhou@amd.com> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>