MdeModulePkg/PciBusDxe: Install PciEnumerationComplete after PciIo
    
    Per PI spec, the PciEnumerationComplete protocol installation
    should be after PciIo installation.
    Today's implementation installs the PciEnumerationComplete
    after hardware enumeration is completed, but before PciIo
    installation.
    The change corrects the spec/implementation gap.
    The change also benefits certain implementation that depends on
    the PciIo handle in PciEnumerationComplete callback.
    
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
    Reviewed-by: Star Zeng <star.zeng@intel.com>