drm/i915: detach/attach get/put pages symmetry

After performing an operation over the page list for a buffer retrieved by
i915_gem_object_get_pages() the pages need to be returned with
i915_gem_object_put_pages(). This was not being observed for the phys
objects which were thus leaking references to their backing pages.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
CC: Dave Airlie <airlied@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_gem.c

@@ -4312,6 +4312,8 @@ void i915_gem_detach_phys_object(struct drm_device *dev,
 	}
 	drm_clflush_pages(obj_priv->pages, page_count);
 	drm_agp_chipset_flush(dev);
+
+	i915_gem_object_put_pages(obj);
 out:
 	obj_priv->phys_obj->cur_obj = NULL;
 	obj_priv->phys_obj = NULL;
@@ -4369,6 +4371,8 @@ i915_gem_attach_phys_object(struct drm_device *dev,
 		kunmap_atomic(src, KM_USER0);
 	}
 
+	i915_gem_object_put_pages(obj);
+
 	return 0;
 out:
 	return ret;