[DisplayList] Don't call Skia Ganesh methods when its not available. (#162345)

This follows the same pattern followed in [Skia
itself](c59d1f0a27/bench/GpuTools.h (L34)).

The QNX builds don't have either Ganesh or Graphite. This will all go
away in Slimpeller builds too.

engine/src/flutter/display_list/skia/dl_sk_canvas.cc

@@ -366,11 +366,13 @@ void DlSkCanvasAdapter::DrawShadow(const DlPath& path,
 }
 
 void DlSkCanvasAdapter::Flush() {
+#if defined(SK_GANESH)
   auto dContext = GrAsDirectContext(delegate_->recordingContext());
 
   if (dContext) {
     dContext->flushAndSubmit();
   }
+#endif  // defined(SK_GANESH)
 }
 
 }  // namespace flutter