drm/nouveau/devinit/nva3-: fix returnvar.cocci warning

Fix the following coccicheck warning:
drivers/gpu/drm/nouveau/nvkm/subdev/devinit/gt215.c:71:5-12:
Unneeded variable: "disable". Return "0ULL" on line 85.

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/20220425114701.7182-1-guozhengkui@vivo.com