x86/PCI: MMCONFIG: add PCI_MMCFG_BUS_OFFSET() to factor common expression

This factors out the common "bus << 20" expression used when computing the
MMCONFIG address.

Reviewed-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>