Fix double-buffering for spi display (it's actually using double-buffering, now)

src/DisplayApp/DisplayApp.cpp

@@ -45,6 +45,9 @@ void DisplayApp::Process(void *instance) {
   NRF_LOG_INFO("DisplayApp task started!");
   app->InitHw();
 
+  // Send a dummy notification to unlock the lvgl display driver for the first iteration
+  xTaskNotifyGive(xTaskGetCurrentTaskHandle());
+
   while (1) {
 
     app->Refresh();

src/DisplayApp/LittleVgl.cpp

@@ -62,6 +62,8 @@ void LittleVgl::InitTouchpad() {
 }
 
 void LittleVgl::FlushDisplay(const lv_area_t *area, lv_color_t *color_p) {
+  ulTaskNotifyTake(pdTRUE, 500);
+
   auto x = area->x1;
   auto y = area->y1;
   auto width = (area->x2-area->x1)+1;