drm/i915/ringbuffer: Move double invalidate to after pd flush

Continuing the fun of trying to find exactly the delay that is
sufficient to ensure that the page directory is fully loaded between
context switches, move the extra flush added in commit 70b73f9a
("drm/i915/ringbuffer: Delay after invalidating gen6+ xcs") to just
after we flush the pd. Entirely based on the empirical data of running
failing tests in a loop until we survive a day (before the mtbf is 10-30
minutes).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107769


References: 70b73f9a ("drm/i915/ringbuffer: Delay after invalidating gen6+ xcs")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904063802.13880-1-chris@chris-wilson.co.uk