tests/amdgpu/vcn: fix the nop command in IBs

Just make them properly i.e. put 0 to the Nop reg

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>