drm/amd/powerplay: gfxoff-seperate the Vega20 case

seperate the Vega20 case from navi10 for gfxoff so that gfxoff
won't be allowed on Vega20

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>