Platforms/RaspberryPi: Fix incorrect display resolution

Fix https://github.com/pftf/RPi4/issues/100

 by swapping RPi DisplayDxe
resolution settings for 640x480 and 800x600, which were incorrectly
mapped to the settings shown in the Setup configuration variable.

Signed-off-by: Samer El-Haj-Mahmoud <samer.el-haj-mahmoud@arm.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>