drm/nouveau/devinit: allow impl to select its own cold-boot method

drivers/gpu/drm/nouveau/core/subdev/devinit/base.c

drivers/gpu/drm/nouveau/core/subdev/devinit/gm107.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nv04.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nv05.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nv10.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nv1a.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nv20.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nv50.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nv84.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nv98.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nva3.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nvaf.c

drivers/gpu/drm/nouveau/core/subdev/devinit/nvc0.c

drivers/gpu/drm/nouveau/core/subdev/devinit/priv.h