drm/i915: POSTING_READ IPS_CTL before waiting for the vblank

Make sure we write to IPS before we actually wait.
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -3386,6 +3386,7 @@ static void hsw_disable_ips(struct intel_crtc *crtc)
 
 	assert_plane_enabled(dev_priv, crtc->plane);
 	I915_WRITE(IPS_CTL, 0);
+	POSTING_READ(IPS_CTL);
 
 	/* We need to wait for a vblank before we can disable the plane. */
 	intel_wait_for_vblank(dev, crtc->pipe);