Reserve enums for cl_ext_image_drm_format_modifier (KhronosGroup#1020)

Change-Id: Id096c7ab542d6fe04a5f7ae7cdbd031755ca15c8

Author: kpet

xml/cl.xml

@@ -2241,7 +2241,8 @@ server's OpenCL/api-docs repository.
         <enum value="0x4232"        name="CL_DEVICE_DOUBLE_FP_ATOMIC_CAPABILITIES_EXT"/>
         <enum value="0x4233"        name="CL_DEVICE_HALF_FP_ATOMIC_CAPABILITIES_EXT"/>
             <unused start="0x4234" end="0x4236" comment="Reserved for cl_ext_image_tiling_control"/>
-            <unused start="0x4237" end="0x423F"/>
+            <unused start="0x4237" end="0x423A" comment="Reserved for cl_ext_image_drm_modifier"/>
+            <unused start="0x423B" end="0x423F"/>
     </enums>
 
     <enums start="0x4240" end="0x424F" name="enums.4240" comment="Reserved for ICD Loader Layers">