xf86: allow DDX driver for GPU/PCI hot-plug

The current X server infrastructure sets modesetting driver as default driver
to handle PCI-hotplug of a GPU device. This prevents the respective DDX driver
(like AMDGPU DDX driver) to take control of the card.

This patch:
- fine-tunes the PCI-hotplug infrastructure to allow the resoective DDX driver
  to be loaded.
- scans and updates the PCI device list before adding the new GPU device
  in platform, so that the association of the platform device and PCI
  device is in place (dev->pdev).

Cc: Alex Deucher <alexandar.deucher@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
4 jobs for push-pci-hotplug in 2 minutes and 3 seconds