disk_cache: assert if a cache entries keys don't match mesa

disk_cache: assert if a cache entries keys don't match mesa

In ef42423e7be9 I enabled the check for release builds however we
still want to assert in debug builds in case of collisions or
just general bugs with the key building/compare code. Otherwise
it will just fail silently effectively disabling the cache.

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>