asahi: Disable tilebuffer write masking optimization

This seems to flake some dEQPs due to some kind of race/UB (which doesn't even always cause the dEQPs to fail due to leeway in the image comparison, since the problem is usually just a few pixels, but it's there).

I spent a bunch of time trying other flags/things, and almost everything changed the bad pixel pattern randomly but nothing fixed it. Let's revisit this one later, since it looks like a pretty deep rabbit hole.

Signed-off-by: Asahi Lina lina@asahilina.net