drm/amd/powerplay: set VCN1 pg only for sienna_cichlid

navy_flounder has one VCN instance, and the work around
is to avoid smu reponse error when setting VCN1 pg for
the chip. It is preferred VCN0 and VCN1 are separated
for the pg setting so better power efficiency can be
achieved.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>