layers: Fix GetDisplayModePropertiesKHR unique_objects code

Add the available DisplayModes to the map for use by CreateDisplayPlaneSurfaceKHR

Change-Id: I94754259069a844bb516d3cf9d4e130473dbc2e3
diff --git a/vk-layer-generate.py b/vk-layer-generate.py
index 13a0d1a..0e459dd 100755
--- a/vk-layer-generate.py
+++ b/vk-layer-generate.py
@@ -1554,7 +1554,8 @@
                                              'DestroyDevice',
                                              'CreateComputePipelines',
                                              'CreateGraphicsPipelines',
-                                             'GetDisplayPlaneSupportedDisplaysKHR'
+                                             'GetDisplayPlaneSupportedDisplaysKHR',
+                                             'GetDisplayModePropertiesKHR'
                                              ]
         # TODO : This is hacky, need to make this a more general-purpose solution for all layers
         ifdef_dict = {'CreateXcbSurfaceKHR': 'VK_USE_PLATFORM_XCB_KHR',