broadcom/compiler: Z write fixes

This series addresses a few issues:

1. There is a hw restriction by which we need to write Z explicitly from the shader if the shader reads Z.
2. We were always disabling EZ testing when we write Z from a shader, but we only really need to do this if the Z write is not a passthrough write from the FEP value (that is, we only need to do this if the original shader was writing Z).