drm/amdgpu:Use register UVD_SCRATCH9 for VCN ring/ib test

Use register UVD_SCRATCH9 for VCN ring/ib test. Since those registers
can't be directly accessed under DPG(Dynamic Power Gate) mode.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>