gallium: Use unpack_rgba() instead of fetch_rgba in translate_generic

gallium: Use unpack_rgba() instead of fetch_rgba in translate_generic

This is the only user of fetch_rgba outside of llvmpipe, and it's in the
fallback path of this fallback path.  Looking at an example of these two
functions, b8g8r8a8's unpack_rgba is 2.7x as long as fetch_rgba.  It feels
reasonable to sacrifice some perf in this already slow (VBO readback, and
a function pointer call per attribute per vertex) path to reduce our
binary size.  And, if I ever finish getting unpack codegen to switch to
rows instead of rects, that factor will go back down.

Saves 40kb of binary on non-llvmpipe gallium drivers.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6305>