radeonsi: only touch first three coordinates in si_prepare_cube_coords

Sourcing coords_arg[4] is actually never correct, since bias is handled
differently in tex_fetch_args anyway.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
1 file changed