drm/i915: POSTING_READ IPS_CTL before waiting for the vblank

Make sure we write to IPS before we actually wait.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>