Platform/RPi/ConfigDxe: Fix a compilation warning

This fixes "WARNING: default value re-defined with different value"
being produced when trying to set default values for the Scaled
VModes in ConfigDxeHii.vfr.

This warning is generated regardless of what the default value is
being set to and since we don't actually care about setting a default
value, as long as the PCD is properly set, we choose to simply remove
the default.

We also ensure that all the PCDs are set to the expected default value.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>