util: Expose rgba unpack/fetch functions as external functions as well.

Like we just did for pack functions for freedreno, it will be useful to be
able to pick out a specific rgba unpack function instead of going through
the table.

Reviewed-by: Jonathan Marek <jonathan@marek.ca>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6307>