drm/amdgpu: skip CG/PG for gfx during S0ix

Not needed as the device is in gfxoff state so the CG/PG state
is handled just like it would be for gfxoff during runtime gfxoff.

This should also prevent delays on resume.

Reworked from Pratik's original patch (Alex)

Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Pratik Vishwakarma <Pratik.Vishwakarma@amd.com>