drm/nouveau/pci: implement generic code for pcie speed change

v2: rename and group functions
v4: change copyright information
    move printing of pcie speeds into oneinit,
    rename all pcie functions to nvkm_pcie_*
    don't try to raise the pcie version when no higher one is supported
v5: revert Copyright changes and rename nvkm_pcie_raise_version to nvkm_pcie_set_version
v6: remove some useless pci_is_pcie checks and rework messages

Signed-off-by: Karol Herbst <nouveau@karolherbst.de>