OvmfPkg/PlatformPei: hoist PciBase assignment above the i440fx/q35 branching

OvmfPkg/PlatformPei: hoist PciBase assignment above the i440fx/q35 branching

In the MemMapInitialization() function, we currently assign PciBase
different values, on both branches of the board type check. Hoist the
PciBase assignment from the i440fx branch in front of the "if". This is a
no-op for the i440fx branch. On the q35 branch, we overwrite this value,
hence the change is a no-op on q35 as well.

This is another refactoring for simplifying the rest of this series.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1814
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1666941
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1701710
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>