PCI: Move pci_assign_unassigned_root_bus_resources()

We need to be able to call pci_bridge_distribute_available_resources()
from this function so move it accordingly to avoid need for forward
declaration.

No functional impact.

Link: https://lore.kernel.org/r/20220905080232.36087-4-mika.westerberg@linux.intel.com


Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>