util/disk_cache: use a helper to compute cache keys

This will allow to hash additional data into the cache keys or even
change the hashing algorithm easily, should we decide to do so.

v2: don't try to compute key (and crash) if cache is disabled

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/shader_cache.cpp

@@ -1331,7 +1331,7 @@ shader_cache_read_program_metadata(struct gl_context *ctx,
       ralloc_asprintf_append(&buf, "%s: %s\n",
                              _mesa_shader_stage_to_abbrev(sh->Stage), sha1buf);
    }
-   _mesa_sha1_compute(buf, strlen(buf), prog->data->sha1);
+   disk_cache_compute_key(cache, buf, strlen(buf), prog->data->sha1);
    ralloc_free(buf);
 
    size_t size;