intel: vc4: v3d: Call nir_lower_flrp less often

In a similar vein as !12061 (merged). I recall @jekstrand and @alyssa expressing some interest in something along these lines on IRC once.

The v3d and vc4 patches have not been run through shader-db or anything. I'm hoping to get some testing from @itoral or @jasuarez or someone who works on those drivers. I won't land those patches without explicit approval from someone who actually works on that hardware / driver. Well... I might land the trivial warning patch. 😉