util: Split the pack/unpack functions out of the format desc.

util: Split the pack/unpack functions out of the format desc.

This gives the compiler a chance to GC pack/unpack functions separate from
the format descriptions.  For drivers that use everything, this is
+10-20kb, while for libvulkan_intel it's -1.3MB.

Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=1048434
Acked-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5826>