drm/amdgpu: use CPU to flush vmhub if sched stopped

otherwse the flush_gpu_tlb will hang if we unload the
KMD becuase the schedulers already stopped

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