drm/nouveau/devinit: fix returnvar.cocci warnings

Fix the following coccicheck warnings:

drivers/gpu/drm/nouveau/nvkm/subdev/devinit/gf100.c:71:5-12:
Unneeded variable: "disable". Return "0ULL" on line 90.
drivers/gpu/drm/nouveau/nvkm/subdev/devinit/gm107.c:35:5-12:
Unneeded variable: "disable". Return "0ULL" on line 44.
drivers/gpu/drm/nouveau/nvkm/subdev/devinit/g98.c:35:5-12:
Unneeded variable: "disable". Return "0ULL" on line 50.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504161003.9245-1-guozhengkui@vivo.com