drm/i915: fixup overlay stolen memory leak

We need to clean up the overlay first, before taking down the
stolen memory allocator.

This regression has been introducec in

commit 8040513870399f1cb032cb8bc805df5042fedcdf
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Nov 15 11:32:29 2012 +0000

    drm/i915: Allocate overlay registers from stolen memory

v2: Rework the patch a bit as suggested by Chris Wilson:
- move the overlay teardown up, into the modeset cleanup
- move the stolen mm takedown into i915_gem_cleanup_stolen

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 1bb6816..f7f4ef1 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -8869,6 +8869,8 @@
 	flush_scheduled_work();
 
 	drm_mode_config_cleanup(dev);
+
+	intel_cleanup_overlay(dev);
 }
 
 /*