i965: Add Gen6 depth stall flushes before disabling depth in BLORP.

i965: Add Gen6 depth stall flushes before disabling depth in BLORP.

We emit these before configuring depth in the normal path, or actually
using the depth buffer in BLORP - we just failed to emit them when
disabling depth altogether.

On Sandybridge, this also requires the post_sync_nonzero flush.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>