DCache: Don't overlay sequential part, remove IDLE

These are changes to the sequential part:

 * The snoop_read_tagmem overlay was not senseful, as there are state
   transitions which need to be overlayed by snoop_hit only. Therefore
   we check both signals individually.

 * The IDLE state is no longer needed, READ state is default.
DCache: Differentiate cpu hits

For easier understanding have tagmem_hit and cpu_hit, where the latter
indicates this hit is actually result for a cpu operation.