drm/i915/gt: Tweak flushes around ivb ppgtt

A small tweak to flush then invalidate appears to improve the
reliability of ppGTT switches on Ivybridge -- but does not improve
hsw/vlv bcs reliability.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191231120857.4014900-1-chris@chris-wilson.co.uk