drm/amd/pm: set VCN pg per instances

When deciding whether to set pg for vcn1, instances
number is more generic than chip name.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>