Disallow hardware mirroring when displays are on different devices

This will revert back to software mirroring, since hardware mirroring
won't work and will result in a black screen.

BUG=b/35583078

Change-Id: Id8496c5966554f5fa866812cd87fc87e3ddd4a92
Reviewed-on: https://chromium-review.googlesource.com/575669Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488779}

ui/display/manager/chromeos/display_configurator.cc

@@ -365,6 +365,13 @@ bool DisplayConfigurator::DisplayLayoutManagerImpl::FindMirrorMode(
     DisplayState* external_display,
     bool try_panel_fitting,
     bool preserve_aspect) const {
+  if (internal_display->display->sys_path() !=
+      external_display->display->sys_path()) {
+    // Hardware mirroring doesn't work between displays on different devices. In
+    // this case we revert to software mirroring.
+    return false;
+  }
+
   const DisplayMode* internal_native_info =
       internal_display->display->native_mode();
   const DisplayMode* external_native_info =