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: 5e9538c12e ("agx: isolate compiler debug flags")
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22891>

src/asahi/compiler/agx_compiler.h

@@ -809,6 +809,8 @@ struct agx_occupancy {
 
 struct agx_occupancy agx_occupancy_for_register_count(unsigned halfregs);
 
+extern int agx_compiler_debug;
+
 #ifdef __cplusplus
 } /* extern C */
 #endif