Kludge up Linux PCI device shims.

. New drm_pci_attach/drm_pci_detach simplify initialization of
autoconf-derived struct pci_dev.

. New linux_pci_dev_init initializes struct pci_dev so that there's
one place where all its fields are listed.  The kludge parameter is,
well, kludgey.  Sorry.

. Replace pci_kludgey_find_dev by pci_get_bus_and_slot with the same
interface as Linux's, but some kasserts requiring it to look for the
one bus/device/function tuple that i915drm is interested in.

. Add pci_get_class which does similarly, for intel_detect_pch.

Later pci_get_bus_and_slot and pci_get_class should be fixed once we
can pass a cookie through PCI bus enumeration and pci_find_device.

(riastradh)