drm/amdgpu: disable gfxoff on vangogh for the moment (v2)

GFXOFF will be enabled once it's verified on real asic.

v2: move check into gfx10 module.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Aaron Liu <aaron.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>