x86/PCI: MMCONFIG: add lookup function

This patch factors out the search for an MMCONFIG region, which was
previously implemented in both mmconfig_32 and mmconfig_64.  No functional
change.

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