dcache: Fix bug with dcbz closely following stores with the same tag

dcache: Fix bug with dcbz closely following stores with the same tag

This fixes a bug where a dcbz can get incorrectly handled as an
ordinary 8-byte store if it arrives while the dcache state machine is
handling other stores with the same tag value (i.e. within the same
set-sized area of memory).  The logic that says whether to include a
new store in the current wishbone cycle didn't take into account
whether the new store was a dcbz.  This adds a "req.dcbz = '0'" factor
so that it does.  This is necessary because dcbz is handled more like
a cache line refill (but writing to memory rather than reading) than
an ordinary store.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>