drm/nouveau/nvkm: move switcheroo from drm

The DRM driver needs to do things in response to vga_switcheroo_client
callbacks, so this move isn't a direct copy+paste.

3 new callbacks are added to nvif_driver_func, which are basically the
same as the switcheroo callbacks, except NVKM handles all the PCI/ACPI
parts, and calls into the DRM driver for the rest.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>