drm/amdgpu: new method to sync ce&de

sync switch buffer scheme with windows kmd for gfx v8,
step2:
Insert 128NOP after&before VM flush to prevent CE vm fault.

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>