cso: Store pointers to struct cso_sampler in struct sampler_info
cso: Store pointers to struct cso_sampler in struct sampler_info

Preparation for following changes, no functional change intended.

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler