i965: Fix cache pollution race during L3 partitioning set-up.

i965: Fix cache pollution race during L3 partitioning set-up.

We need to split the stalling flush from the RO cache invalidation
into a different PIPE_CONTROL command to make sure that the top of the
pipe invalidation happens after any previous rendering is complete.
Otherwise it's possible for previous rendering to pollute the L3 cache
in the short window of time between RO invalidation and the completion
of the stalling flush.  Fixes rendering artifacts on Unigine Heaven,
Metro Last Light Redux and Metro 2033 Redux.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93540
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93599
Tested-by: Darius Spitznagel <d.spitznagel@goodbytez.de>
Tested-by: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>