x86/pci: Reducde #ifdeffery in PCI init code

Adding a function call before the first #ifdef in arch_pci_init() triggers
a 'mixed declarations and code' warning if PCI_DIRECT is enabled.

Use stub functions and move the #ifdeffery to the header file where it is
not in the way.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200826112332.767707340@linutronix.de