Mem: Fix issue with prefetches originating at non-L1 caches getting stale data

Mem: Fix issue with prefetches originating at non-L1 caches getting stale data

Prefetch requests issued from the L2 or below wouldn't check if valid data is
present higher in the system. If a prefetch into the L2 occured at the same
time as writeback from a higher-level cache the dirty data could be replaced
in by unmodified data in memory.