drm/nouveau/mc/ga100: make ga100_mc_device static

Make ga100_mc_device static as it isn't exported, to
fix the following sparse warning:

drivers/gpu/drm/nouveau/nvkm/subdev/mc/ga100.c:51:1: warning: symbol 'ga100_mc_device' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221229155249.669436-1-ben-linux@fluff.org