pan/bi: Ignore signedness in vertex fetch

pan/bi: Ignore signedness in vertex fetch

We just want a bit-exact transfer for integers. Using .auto32 accomplishes this
without any clamping shenanigans. Fixes gl-3.0-vertexattribipointer.

Note we can't use .auto32 unconditionally, since reading a uint vertex as float
is supposed to convert (or something like that, gl-2.0-vertexattribpointer tests
the bad case at any rate).

Fixes: 482cc273af5 ("pan/bi: Implement load attribute with the builder")
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21891>