drm/amdgpu/vcn: Share vcn_v2_0_dec_ring_test_ring to vcn2.5

Share vcn_v2_0_dec_ring_test_ring to vcn2.5 to support
vcn software ring.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>