D$: fix perf.blocked hint signal

d_address_inc is only valid when d.valid is true (or, more precisely, when
d.bits.opcode is valid).  So, we weren't unblocking the core on the most
optimal cycle in some cases.  This only seems to happen in practice with
noisy drops, since there d.bits.opcode is sometimes ReleaseAck, whereas
otherwise d.bits.opcode is almost always GrantData.

Not a correctness issue.