Platform/AMD/OverdriveBoard: use default resolution for GOP

Set the GOP resolution to 0x0 so that the resolution will be
chosen by the driver, usually based on the capabilities of
the connected display.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>