panfrost: Identify "Base vertex offset" signedness

panfrost: Identify "Base vertex offset" signedness

This is signed, not unsigned. We were already passing negatives and silently
relying on 2's complement and C to do the right thing. But that's silly. We
should just, actually do the right thing.

Found while struggling to debug primitive-restart-draw-mode.

v2: Update the other architectures too, including a decode_csf.c change for the
v10 incarnation of this v4-era field.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net> [v1]
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com> [v1]
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21891>