PCI: Refactor pbus_size_mem()

The original idea comes from Ram Pai.  This patch puts the chunk of
code for calculating the minimal alignment of memory window into a
separate inline function.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>