Use acpi_pci_power_manageable() instead of duplicating the logic in
acpi_pci_bridge_d3().  No functional change intended.

[bhelgaas: split out from
https://lore.kernel.org/r/20210817180500.1253-8-ameynarkhede03@gmail.com]
Signed-off-by: Shanker Donthineni <sdonthineni@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>