AmpereAltraPkg: Remove PCIe "DevMapModeAuto" case in retrieving Devmap mode

Because PCIe Auto bifurcation feature doesn't depend on the default
Devmap mode so that "DevMapModeAuto" is unnecessary in retrieving
the PCIe Devmap default mode.

Signed-off-by: Minh Nguyen <minhnguyen1@os.amperecomputing.com>
Reviewed-by: Tinh Nguyen <tinhnguyen@os.amperecomputing.com>
Reviewed-by: Nhi Pham <nhi@os.amperecomputing.com>