drm/nouveau: Do not try to disable PCI device on Tegra

When Nouveau is instantiated on top of a platform device, the dev->pdev
field will be NULL and calling pci_disable_device() will crash. Move the
PCI disabling code to the PCI specific driver removal code.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>