drm/amdgpu/vcn: drop extra cancel_delayed_work_sync()

We already call this in the hw_fini() methods for all
VCN instances, so no need to call it again in
amdgpu_vcn_suspend().

Tested-by: David (Ming Qiang) Wu <David.Wu3@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>