panfrost: Disable CRC by default

Known unsound code.

So far I'm not convinced transaction elimination is doing us much good. Even in synthetic glmark style benchmarks this seems to be a few % hit at most. Given that transaction elimination is unsound by design, and that panfrost's implementation is buggy in several places and getting it right (up to the unsoundness of the hardware feature itself) would take actual engineering effort, and the priority is making glamor work... disabling is the obvious choice here.

For now, we leave the code but gate it behind a env var flag (PAN_MESA_DEBUG=crc) rather than defaulting to enabled unless PAN_MESA_DEBUG=nocrc is set. This way, we can still experiment with it if we need that data ("what performance could we gain if we had this feature, unsoundness be damned?"). That said, I'm not really ok with having unsoundness on my devices, y'know? Back of the napkin math suggests that it's not unlikely that somebody has hit a transaction elimination collision in the wild with the DDK.

Boils down to values.

Closes: #8113 (closed) Signed-off-by: Alyssa Rosenzweig alyssa@collabora.com