i965: Implement WaCsStallAtEveryFourthPipecontrol on IVB/BYT.

i965: Implement WaCsStallAtEveryFourthPipecontrol on IVB/BYT.

According to the documentation, we need to do a CS stall on every fourth
PIPE_CONTROL command to avoid GPU hangs.  The kernel does a CS stall
between batches, so we only need to count the PIPE_CONTROLs in our batches.

v2: Get the generation check right (caught by Chris Wilson),
    combine the ++ with the check (suggested by Daniel Vetter).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>