PCI: Split out bridge window override of minimum allocation address

pci_bus_alloc_resource() avoids allocating space below the "min" supplied
by the caller (usually PCIBIOS_MIN_IO or PCIBIOS_MIN_MEM).  This is to
protect badly documented motherboard resources.  But if we're allocating
space inside an already-configured PCI-PCI bridge window, we ignore "min".

See 688d1918 ("pci: make bus resource start address override minimum IO
address").

This patch moves the check to make it more visible and simplify future
patches.  No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>