layers: Fix unique_objects creation of VkDisplayKHR

Make an explict_GetPhysicalDeviceDisplayProperties.

Change-Id: I39411d0dd97ca03dbf42d7e79b02243ef8988078
diff --git a/vk-layer-generate.py b/vk-layer-generate.py
index 4d01549..977fd13 100755
--- a/vk-layer-generate.py
+++ b/vk-layer-generate.py
@@ -1556,6 +1556,7 @@
                                              'DestroyDevice',
                                              'CreateComputePipelines',
                                              'CreateGraphicsPipelines',
+                                             'GetPhysicalDeviceDisplayPropertiesKHR',
                                              'GetDisplayPlaneSupportedDisplaysKHR',
                                              'GetDisplayModePropertiesKHR'
                                              ]