The current Marlin 2.0.x firmware cannot properly access some of the pins on the EXP2 connector. In order to use the RepRap Discount Full Graphic Smart Controller you'll need a custom cable that gets three of the signals from a different connector. In that cable move the pin/wire that:
used to go to P0.8 to J8-3
" " " " P0.7 to J8-2
" " " " P0.5 to J8-4
If pins different than the J8 ones above are used then the spi_pins.h file will need to be modified.
An octopus cable something like the Adafruit 1199 will simply the construction of the custom cable. Just plug the Adafruit 1199 into one of the cables that came with the LCD and the individual pins into J8 and EXP2 as needed.