st7789: Avoid static flash on LCD init.

drivers/st7789/st7789.cpp

@@ -112,9 +112,6 @@ namespace pimoroni {
 
     command(reg::INVON);   // set inversion mode
     command(reg::SLPOUT);  // leave sleep mode
-    command(reg::DISPON);  // turn display on
-
-    sleep_ms(100);
 
     configure_display(rotation);
 
@@ -314,6 +311,12 @@ namespace pimoroni {
 
       gpio_put(cs, 1);
     }
+
+    if(!display_on) {
+      command(reg::DISPON);  // turn display on
+      sleep_ms(100);
+      display_on = true;
+    }
   }
 
   void ST7789::set_backlight(uint8_t brightness) {

drivers/st7789/st7789.hpp

@@ -43,6 +43,7 @@ namespace pimoroni {
     PIO parallel_pio;
     uint parallel_offset;
     uint st_dma;
+    bool display_on = false;
 
 
     // The ST7789 requires 16 ns between SPI rising edges.