tu6_vpc_varying_mode returned how many bits are consumed IF they are non-zero, for SMOOTH mode nothing is written and it was treated like no bits were consumed.
When input with smooth interpolation was the last one and straddled the VPC_VARYING_INTERP_MODE regs, the last interp mode were not written.
Fixes misrendering in "Psychonauts 2".
Cc: mesa-stable
Force same value for flat.b sources along the way. And improved printing of FS varyings.