i965: Enable the PIPE_CONTROL workaround workaround out of paranoia.

i965: Enable the PIPE_CONTROL workaround workaround out of paranoia.

There's scary stuff going on in PIPE_CONTROL internals, and if the
BSpec says to do this to make PIPE_CONTROL work, I'll go ahead and do
it because we'll probably never be able to debug it after the fact.

v2: Use stall at scoreboard instead of depth stall, as noted by Ken.