zink: do not ask glsl-compiler to unroll

We don't really need loops unrolled, so let's just disable this. This is generally recommended for NIR drivers, but we can do even better; not even unroll in NIR. And since we don't set nir_shader_compiler_options::max_unroll_iterations, we're already there.

This turns out to be more difficult than first assumed, due to what appears to be some Lavapipe issues. Let's see if we can fix those.

This requires a fix in Lavapipe first, to avoid regressing piglits.