Revert "x86/PCI: MMCONFIG: Check earlier for MMCONFIG region at address zero"

Revert "x86/PCI: MMCONFIG: Check earlier for MMCONFIG region at address zero"

This reverts commit 40937dca6f3ef49eaecd81e44e8a2fc6ca0e0d70.

40937dca was intended to be a cleanup that didn't change anything, but in
fact, for systems without _CBA (which is almost everything), it broke
extended config space for domain 0 and all config space for other domains.

Reference: http://lkml.kernel.org/r/20131004011806.GE20450@dangermouse.emea.sgi.com
Reported-by: Hedi Berriche <hedi@sgi.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>