x86, acpi, pci: Reorder logic of pci_mmconfig_insert() function

x86, acpi, pci: Reorder logic of pci_mmconfig_insert() function

This patch is the first step for MMCONFIG refactoring process.

Code that uses pci_mmcfg_lock will be moved to common file and become
accessible for all architectures. pci_mmconfig_insert() cannot be moved
so easily since it is mixing generic mmcfg code with x86 specific logic
inside of mutual exclusive block guarded by pci_mmcfg_lock.

To get rid of that constraint we reorder actions as fallow:
1. mmconfig entry allocation can be done at first, does not need lock
2. insertion to iomem_resource has its own lock, no need to wrap it into mutex
3. insertion to mmconfig list can be done as the final stage in separate
function (candidate for further factoring)

Signed-off-by: Tomasz Nowicki <tomasz.nowicki@linaro.org>
Tested-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Vadim Lomovtsev <Vadim.Lomovtsev@caviumnetworks.com>