pan/bi: Add FAU update helper

pan/bi: Add FAU update helper

This comes in destructive and nondestructive flavours, to be used to
insert an instruction into a tuple and check if an instruction is
insertable respectively. It is responsible for FAU slot matching.

It's annoying this sort of logic is duplicated in 3 places
(bi_lower_fau, here, and packing) but they each work with different sets
of assumptions...

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/8354>