fix driver issues, add simple display testing code

Core/Inc/stm32f4xx_it.h

@@ -55,6 +55,7 @@ void SVC_Handler(void);
 void DebugMon_Handler(void);
 void PendSV_Handler(void);
 void SysTick_Handler(void);
+void EXTI0_IRQHandler(void);
 /* USER CODE BEGIN EFP */
 
 /* USER CODE END EFP */