drm/nouveau: merge nouveau_platform.ko into nouveau.ko

Having the two modules separated causes various unneeded complications,
including having to export symbols accessed between the modules. Make
things simpler by compiling platform device support into nouveau.ko.
Platform device support remains optional and is only compiled on Tegra.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Vince Hsu <vinceh@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>