winsys/amdgpu: use pb_cache buckets for fewer pb_cache misses
winsys/amdgpu: use pb_cache buckets for fewer pb_cache misses

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler