Update graphicstest_tft_gizmo.ino

[mikeysklar]

Minor change. Turn on backlight AFTER display is initialized.

Based on github issue

Forum issues:
TFT Gizmo not working with Arduino sketch

noobie needs help with TFT Gizmo!

Test and run on Gizmo + CPB using hardware SPI or software SPI.

* 0.9.0 bootloader
* Arduino 2.3.2
* Adafruit nRF52 BSP 1.6.1

Using library Adafruit GFX Library at version 1.11.9 in folder: /Users/sklarm/Documents/Arduino/libraries/Adafruit_GFX_Library 
Using library Adafruit BusIO at version 1.16.0 in folder: /Users/sklarm/Documents/Arduino/libraries/Adafruit_BusIO 
Using library Wire at version 1.0 in folder: /Users/sklarm/Library/Arduino15/packages/adafruit/hardware/nrf52/1.6.1/libraries/Wire 
Using library SPI at version 1.0 in folder: /Users/sklarm/Library/Arduino15/packages/adafruit/hardware/nrf52/1.6.1/libraries/SPI 
Using library Adafruit ST7735 and ST7789 Library at version 1.10.3 in folder: /Users/sklarm/Documents/Arduino/libraries/Adafruit_ST7735_and_ST7789_Library 
Using library Adafruit TinyUSB Library at version 3.1.4 in folder: /Users/sklarm/Documents/Arduino/libraries/Adafruit_TinyUSB_Library