Platform/RaspberryPi: default DisplayDxe to just native mode

The scaled resolutions are useful, but are not the default expected
by most users. Linux and BSDs don't set preferred resolution in
their OS loader, so when booting via setup UI, the OS is left
running at 800x600, not the native resolution. This looks crummy.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Tested-by: Pete Batard <pete@akeo.ie>