asahi: Fix disk cache disable with AGX_MESA_DEBUG
We go to initialize the disk cache before we've compiled any shaders so agx_compiler_debug is 0 at this point. Don't try to read it, instead go through sa safe getter that will do the right thing.
Fixes: 5e9538c1 ("agx: isolate compiler debug flags") Signed-off-by: Alyssa Rosenzweig alyssa@rosenzweig.io