disk_cache: move cache item loading code into disk_cache_load_item() helper

This should be helpful if someone chooses to implement cache support on
windows. Also providing this greater level of abstraction makes it easier
to implement alterative cache layouts in future.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6197>

src/util/disk_cache_os.h

@@ -103,6 +103,9 @@ disk_cache_evict_lru_item(struct disk_cache *cache);
 void
 disk_cache_evict_item(struct disk_cache *cache, char *filename);
 
+void *
+disk_cache_load_item(struct disk_cache *cache, char *filename, size_t *size);
+
 char *
 disk_cache_get_cache_filename(struct disk_cache *cache, const cache_key key);