llvmpipe: Fix perspective divide interpolation.

llvmpipe: Fix perspective divide interpolation.

Intuition != mathematics, so this time I actually worked out the right
formula for first order approximation of perspective interpolation.

Ironically, per quad divide actually makes things slower when compared
with per pixel divide -- probably because the divide hardware unit is
rarely used, whereas the multiply unit is typically already saturated
and the first order approximation imply more multiplications.