amd/amdgpu: Disable VCN DPG mode for Picasso

Concurrent operation of VCN and JPEG decoder in DPG mode is
causing ring timeout due to power state.

Signed-off-by: Veerabadhran Gopalakrishnan <veerabadhran.gopalakrishnan@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>