gallium: lower raw 64-bit vertex formats in cso/vbuf instead of st/mesa

gallium: lower raw 64-bit vertex formats in cso/vbuf instead of st/mesa

This lower each 64-bit vertex element into 1 or 2 32-bit vertex elements.
The motivation is to simplify st/mesa, which uses a lot more complicated
code to do the same thing, and do the lowering only on a cso_cache miss.

Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11370>