radeonsi: refactor si_update_vgt_shader_config

We'll have to extend this at some point, and using a bitfield union in
this way makes it easier to get the right index without excessive
branching.

Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
2 files changed