WIP: freedreno: Don't do texture prefetch for samples after a discard.

My theory is that it's more important that we avoid the memory traffic on pixels that would be discarded than that we pipeline the fetches better.

I'm seeing lots of shaders affected in shader-db (ETQW, GLBenchmark 2.7 (t-rex), tesseract), so hopefully this will show up easily in benchmarks, but it's not doing anything significant in my renderdoc-traces collection so far.