drm/amdgpu: increase CGCG gfx idle threshold for Arcturus

Follow the hw spec, and no need to consider gfxoff on Arcturus

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>