i965: add CS stall on VF invalidation workaround

Even with the previous commit, hangs are still happening. The problem
there is that the VF cache invalidate do happen immediately without
waiting for previous rendering to complete. What happens is that we
invalidate the cache the moment the PIPE_CONTROL is parsed but we
still have old rendering in the pipe which continues to pull data into
the cache with the old high address bits. The later rendering with the
new high address bits then doesn't have the clean cache that it
expects/needs.

v2: Update commit message/explanation with Jason's

Signed-off-by: Lionel Landwerlin <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>
Fixes: a363bb2cd0e2a1 ("i965: Allocate VMA in userspace for full-PPGTT systems.")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109072

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