anv: Export filter_minmax support only when it's really supported

Fixes: bea4d4c78c ("anv: add VK_EXT_sampler_filter_minmax support")

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3071>

src/intel/vulkan/anv_device.c

          VkPhysicalDeviceSamplerFilterMinmaxPropertiesEXT *properties =
             (VkPhysicalDeviceSamplerFilterMinmaxPropertiesEXT *)ext;
          properties->filterMinmaxImageComponentMapping = pdevice->info.gen >= 9;
-         properties->filterMinmaxSingleComponentFormats = true;
+         properties->filterMinmaxSingleComponentFormats = pdevice->info.gen >= 9;
          break;
       }