drm/amdgpu: enable gfxoff feature for navi10 asic

enable gfxoff feature for some navi10 asics

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>