OvmfPkg/PlatformPei: assign PciSize on both i440fx/q35 branches explicitly

OvmfPkg/PlatformPei: assign PciSize on both i440fx/q35 branches explicitly

In the MemMapInitialization() function, we currently have a common
PciSize assignment, shared between i440fx and q35. In order to simplify
the rest of this series, lift and duplicate the assignment identically to
both board-specific branches.

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>