dcache: Ease timing on wishbone data and byte selects

dcache: Ease timing on wishbone data and byte selects

This eliminates a path where the inputs to r1.wb.dat and r1.wb.sel
depend on req_op, which depends on the TLB and cache hit detection.
In fact they only need to depend on the nature of the request in
r0.req (i.e. DCBZ, store, cacheable load, or non-cacheable load).
This sets them at the beginning of the code for IDLE state rather
than inside the req_op case statement.

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