linuxkpi: Fix `pci_upstream_bridge()` with DRM devices

[dumbbell]

Summary

In the case of DRM drivers, the passed device is a child of vgapci. We want to start the lookup from vgapci, so the parent of the passed drmn. We use the isdrm flag to determine if we are in this situation.

This fixes an infinite loop with the amdgpu DRM driver that started to use this function in Linux 6.8: pci_upstream_bridge() was returning itself and the code in amdgpu was calling it again, hoping to get a device with a vendor that is not "ATI".

This is part of the update of DRM drivers to Linux 6.8.

Sponsored by: The FreeBSD Foundation

[bz]

sys/compat/linuxkpi/common/include/linux/pci.h

532

If you just want to add the one extra skip, is that .. as initialized by linux_pci_register_driver()?

if (pdev->pdrv != NULL && pdev->pdrv->isdrm)
     bridge = device_get_parent(bridge);

[dumbbell]

Address comment from @bz.