pan/bi: Mark mod to string as maybe unused
Fixes warnings like the following under clang: src/panfrost/bifrost/bi_printer.c:599:1: warning: unused function 'bi_widen_as_str' [-Wunused-function] bi_widen_as_str(enum bi_widen widen) Signed-off-by:Alyssa Rosenzweig <alyssa@collabora.com> Part-of: <mesa/mesa!12003>