drm/amdgpu: enable athub/mmhub PG for navy_flounder

Enable athub/mmhub PG by setting the corresponding flags.
Actually the enablement is exercised by PMFW.

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>