v3d: Fix shaders using pixel center W but no varyings.

v3d: Fix shaders using pixel center W but no varyings.

The docs called this field "uses both center W and centroid W", but
actually it's "do you need center W even if varyings don't obviously call
for it?"

Fixes dEQP-GLES3.functional.shaders.builtin_variable.fragcoord_w