gallium/indices: introduce u_primconvert_config

gallium/indices: introduce u_primconvert_config

This struct allows us to pass the configuration as a struct, which can
more easily be extended to take more arguemnts as long as we're careful
about zero-initialization.

We keep the old create-function, but implement it as a wrapper on top so
we don't have to update all existing call-sites right now.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5976>