tu: Fix varyings interpolation reading stale values
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
Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20533>