PCI/DPC: Enable DPC only if AER is available

The "Determination of DPC Control" implementation note in PCIe r4.0, sec
6.1.10, recommends the operating system always link DPC control to the
control of AER, as the two functionalities are strongly connected.

To avoid conflicts over whether platform firmware or the OS controls DPC,
enable DPC only if AER is enabled in the OS, and the device's error
handling does not have firmware-first AER handling.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>