drm/amdgpu: enable DPG mode for Navi12

Enable Dynamic Power Gating VCN for Navi12.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>