util/disk_cache: allow drivers to pass a directory structure

util/disk_cache: allow drivers to pass a directory structure

In order to avoid costly fallback recompiles when cache items are
created with an old version of Mesa or for a different gpu on the
same system we want to create directories that look like this:

./{TIMESTAMP}_{LLVM_TIMESTAMP}/{GPU_ID}

Note: The disk cache util will take a single timestamp string, it is
up to the backend to concatenate the llvm string with the mesa string
if applicable.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>