mesa/main: fix distance attenuation calculation in ffvertex
What does this MR do and why?
Distance attenuation is miscalculated when ffvertex is changed to emit NIR directly. The dist parameter to calculate_light_attenuation() is the reciprocal of ||VP|| used in the distance attenuation formula (2.4). So we need to get its reciprocal first before applying it to the distance attenuation formula.