pan/bi: Fix M1/M2 decoding in disassembler

pan/bi: Fix M1/M2 decoding in disassembler

C's definition of the % operator has a footgun around sign conversion.
Avoid it and just use bitwise arithemtic instead like the hardware
would, fixing the disassembly and making buggy assembly more obvious.

Fixes: 08a9e5e3e89 ("pan/bi: Decode M values in disasm")
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8723>