docs(porting) clarify that displays must be registered before input devices
Related: (thanks
## Types of input devices
-To register an input device an `lv_indev_drv_t` variable has to be initialized:
+To register an input device an `lv_indev_drv_t` variable has to be initialized. **Be sure to register at least one display before you register any input devices.**
```c
+lv_disp_drv_register(&disp_drv);
+
static lv_indev_drv_t indev_drv;
lv_indev_drv_init(&indev_drv); /*Basic initialization*/
indev_drv.type =... /*See below.*/
