i965/sched: write-after-read dependencies are free

i965/sched: write-after-read dependencies are free

Although write-after-write dependencies have the same latency as
read-after-write dependencies due to how the register scoreboard works,
write-after-read dependencies aren't checked by the EU at all, so
they're purely a constraint on how the scheduler can order the
instructions.

v2: fix accumulator dependencies too.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>