drm/nouveau: Start using new drm_dev initialization helpers

Per the documentation in drm_get_pci_dev(), this function is deprecated
and shouldn't be used anymore. As it turns out, we're going to need to
stop using drm_get_pci_dev() anyway in order to allow us to turn off the
card before full system shutdowns, otherwise we'll hit race conditions
with userspace while trying to tear down the card on shutdown.

So, start using drm_dev_get() and drm_dev_put(), and just turn our
load/unload callbacks into open coded init/fini() functions.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>