Vulkan display mode: fix skia build failure

Separate angle_use_vulkan_display into another
declare_args() for skia build.

Bug: angleproject:5214
Change-Id: I9f829b8a8c95947832f69ca2d78e050aa60ce8ae
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2519171
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

gni/angle.gni

@@ -39,11 +39,14 @@ if (angle_has_build) {
     build_with_chromium = false
     dcheck_always_on = false
     angle_use_x11 = (is_linux || is_chromeos) && !is_ggp
-    angle_use_vulkan_display = is_linux && !angle_use_x11 && !is_ggp
     use_ozone = false
     ozone_platform_gbm = false
     use_fuzzing_engine = false
   }
+
+  declare_args() {
+    angle_use_vulkan_display = is_linux && !angle_use_x11 && !is_ggp
+  }
 }
 
 # Subdirectory to place data files (e.g. layer JSON files).