drm/nouveau/mmu: Remove unneeded semicolon

Fixes coccicheck warning:

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.h:307:2-3: Unneeded semicolon
drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c:583:2-3: Unneeded semicolon

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>