AMDGPU: Move handling of allocation of fixed ABI inputs

For the fixed ABI, set this in the initial argument constructor,
rather than relying on the allocation logic to set the values. Also
stop passing them for amdgpu_gfx, since the DAG path seems to skip
these. I'm unclear on what amdgpu_gfx's expectations are.  This will
allow moving the special input registers out of the normal argument
range.