drm/amdgpu:fix virtual dce bug

this fix the issue that access memory after freed
after driver unloaded.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>