Move pci_enable_acs() and dependencies further up in the source code to
avoid having to forward declare it when we make it static in near future.

No functional changes intended.

Link: https://lore.kernel.org/r/20200707224604.3737893-1-rajatja@google.comSigned-off-by: Rajat Jain <rajatja@google.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>