drm/i915: vlv: fix gunit HW state corruption during S4 suspend

During S4 freeze we don't call intel_suspend_complete(), which would
save the gunit HW state, but during S4 thaw/restore events we call
intel_resume_prepare() which restores it, thus ending up in a corrupted
HW state.

Fix this by calling intel_suspend_complete() from the corresponding
freeze_late event handler.

The issue was introduced in
commit 016970be
Author: Sagar Kamble <sagar.a.kamble@intel.com>
Date:   Wed Aug 13 23:07:06 2014 +0530

CC: Sagar Kamble <sagar.a.kamble@intel.com>
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>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>