drm/i915: add poweroff_late handler

The suspend_late handler saves some registers and powers off the device,
so it doesn't have a big overhead. Calling it at S4 poweroff_late time
makes the power off handling identical to the S3 suspend and S4 freeze
handling, so do this for consistency.
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -1548,6 +1548,7 @@ static const struct dev_pm_ops i915_pm_ops = {
 	.thaw_early = i915_pm_resume_early,
 	.thaw = i915_pm_resume,
 	.poweroff = i915_pm_suspend,
+	.poweroff_late = i915_pm_suspend_late,
 	.restore_early = i915_pm_resume_early,
 	.restore = i915_pm_resume,
 	.runtime_suspend = intel_runtime_suspend,