linuxkpi: handle ARI

Stop trying to manually calculate RID, which cannot be done correctly
by PCI_DEVFN(). Use PCI_GET_RID() method instead.

Do not use pci_find_dbsf() to go from the linux pci_dev to freebsd
device_t. First, device is readily available as dev.bsddev. Second,
using pci_find_dbsf() fails for ARI-enabled functions with large
function numbers, because PCI_SLOT()/PCI_FUNC() are for non-ARI.

Reviewed by: bz, hselasky, manu
Tested by: manu (drm)
Sponsored by: Mellanox Technologies/NVidia Networking
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27960