drm/amdgpu/vcn: use dummy register selects AID for VCN_RAM ucode

Use dummy register 0xDEADBEEF selects AID for PSP VCN_RAM ucode.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Sonny Jiang <sonny.jiang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>