radeonsi: reduce the number of generics for shader IO unique indices

radeonsi: reduce the number of generics for shader IO unique indices

This is a high as possible while still allowing to merge the bitfields
with the next commit.

For OpenGL, 32 would be sufficient. Nine apparently uses (much!) higher
indices than. Indices that are out of bound don't hurt for VS-PS
pipelines, except that the VS output kill optimization is not applied.

Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>