From the AHB spec:
"A slave only has to provide valid data when a transfer completes with an OKAY
response. ERROR responses do not require valid read data."
We had planned for a while to add an 'Overwrite' message which obtains
permissions without requiring retrieval of data. This is useful whenever
a master knows it will completely replace the contents of a cache block.
Instead of calling it Overwrite, we decided to split the Acquire type.
If you AcquirePerm, you MUST Release and ProbeAck with Data.
If a multibeat response comes back, the source might be reused.
If response reordering has made the multibeat response invalid,
we need to remember this even if the valid bit is cleared on reuse.
