drm/i915: add missing unregister_oom_notifier to the error/unload path

I'm trying to reduce the WARNs during driver reload and this was one of
them. Also while at it remove the redundant condition from before
unregister_shrinker().

v2:
- fix the error path too and move the unregister to its logical place
(Chris)
- remove redundant condition from before unregister_shrinker()
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_dma.c

@@ -44,6 +44,7 @@
 #include <acpi/video.h>
 #include <linux/pm.h>
 #include <linux/pm_runtime.h>
+#include <linux/oom.h>
 
 #define LP_RING(d) (&((struct drm_i915_private *)(d))->ring[RCS])
 
@@ -1741,7 +1742,7 @@ int i915_driver_load(struct drm_device *dev, unsigned long flags)
 	intel_power_domains_remove(dev_priv);
 	drm_vblank_cleanup(dev);
 out_gem_unload:
-	if (dev_priv->mm.shrinker.scan_objects)
+	WARN_ON(unregister_oom_notifier(&dev_priv->mm.oom_notifier));
 	unregister_shrinker(&dev_priv->mm.shrinker);
 
 	if (dev->pdev->msi_enabled)
@@ -1793,7 +1794,7 @@ int i915_driver_unload(struct drm_device *dev)
 
 	i915_teardown_sysfs(dev);
 
-	if (dev_priv->mm.shrinker.scan_objects)
+	WARN_ON(unregister_oom_notifier(&dev_priv->mm.oom_notifier));
 	unregister_shrinker(&dev_priv->mm.shrinker);
 
 	io_mapping_free(dev_priv->gtt.mappable);