i965: Implement a CS stall workaround on Broadwell.

According to the latest documentation, any PIPE_CONTROL with the
"Command Streamer Stall" bit set must also have another bit set,
with five different options:

   - Render Target Cache Flush
   - Depth Cache Flush
   - Stall at Pixel Scoreboard
   - Post-Sync Operation
   - Depth Stall

I chose "Stall at Pixel Scoreboard" since we've used it effectively
in the past, but the choice is fairly arbitrary.

Implementing this in the PIPE_CONTROL emit helpers ensures that the
workaround will always take effect when it ought to.

Apparently, this workaround may be necessary on older hardware as well;
for now I've only added it to Broadwell as it's absolutely necessary
there.  Subsequent patches could add it to older platforms, provided
someone tests it there.

v2: Only flag "Stall at Pixel Scoreboard" when none of the other bits
    are set (suggested by Ian Romanick).

v3: Prefix the function with "gen8" (requested by Eric).

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ian Romanick <[email protected]> (v2)
Reviewed-by: Eric Anholt <[email protected]>

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