drm/amdgpu: add the trailing fence per ring

The trailing fence for ring is used to track the
completion of preemption.

Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>