What register or setting can control the brightness of an ST7735 based 1.8" TFT Display?
There is no setting on the ST7735 that allows you to control the brightness.

You must PWM the LED backlight on your TFT display in order to control the brightness. 

See the ST7735 datasheet here: https://www.displayfuture.com/Display/datasheet/controller/ST7735.pdf