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:
1. fine-tunes the PCI-hotplug infrastructure to allow the respective DDX driver
to be loaded.
2. 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).

V2:
- Fixed typo in commit message (Martin)
- Added R-B from Adam.
- Added ACK from Alex and Martin.

V3:
- Added an output class based approach for finding the DDX driver (Aaron)

Cc: Alex Deucher <alexandar.deucher@amd.com>

Suggested-by: Aaron Plattner <aplattner@nvidia.com> (v3)
Reviewed-by: Adam Jackson <ajax@redhat.com>(v1)
Acked-by: Martin Roukala <martin.roukala@mupuf.org>(v1)
Acked-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
4 jobs for push-pci-hotplug in 1 minute and 46 seconds (queued for 1 second)
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