anv: implement gen9 post sync pipe control workaround

anv: implement gen9 post sync pipe control workaround

We've been missing this workaround for a while and since it's required
for Gen12, let's implement it for Gen9 first.

v2: Update comment for Gen9.

v3: Fix clearing of bits... (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3405>