nir: fix lower_clip pass for freedreno
All of this was failing on a4xx. Not sure about other platforms (a5xx+ doesn't need the frag lowering, and a3xx doesn't have piglit runs in CI).
There are a couple more freedreno-specific fixes that are needed, but those will be addressed in another series. (We also need to sort out compact arrays there, since ir3's support for them is ... lacking.)