tests/amdgpu/vcn: add VCN2.0 decode support

With different register offsets from VCN1.0

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>