i965/sklgt4: Implement depth/timestamp write w/a

The stated bug describes a scenario in which a post sync write operation for
depth or timestamp can be ignored. There are two workarounds suggested, the
first and easier is to simply do a cs stall when we do these type of writes.
The second option is to do a PIPE_CONTROL flush after the post sync but before
the data is required.

Generally, I believe the data written out is consumed by the application on the
CPU side and so doing the easier of the two is ideal. Furthermore, these queries
aren't tremendously common in the perf sensitive apps I have looked at. However,
there could be cases where a shader stage might directly consume the data, and
as a result option 2 may be desirable.

This patch goes with the easier solution for now.

gen9lp bug_de_id=2137196

By itself, this does *not* fix any of the GT4 hangs we're currently
experiencing.

Cc: Mika Kuoppala <[email protected]>
Signed-off-by: Ben Widawsky <[email protected]>
Reviewed-by: Anuj Phogat <[email protected]>

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