drm/amdgpu: use sched fence if possible

when preemption feature lands, the SA bo should rely on sched
fence, because hw fence will be invalid after its job preempted
or skipped.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>