drm/nouveau/gr/gm200: s/gm204/gm200/

Most of the per-chipset differences will go away when we fully switch
to using the register lists provided by the firmware files, which will
leave all the remaining code "belonging" to GM200.

This is a preemptive rename from GM204 to GM200.

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