drm/nouveau: fix build error without CONFIG_IOMMU_API

If CONFIG_IOMMU_API is n, build fails:

vers/gpu/drm/nouveau/nvkm/subdev/ltc/gp10b.c:37:9: error: implicit declaration of function dev_iommu_fwspec_get; did you mean iommu_fwspec_free? [-Werror=implicit-function-declaration]
  spec = dev_iommu_fwspec_get(device->dev);
         ^~~~~~~~~~~~~~~~~~~~
         iommu_fwspec_free
drivers/gpu/drm/nouveau/nvkm/subdev/ltc/gp10b.c:37:7: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
  spec = dev_iommu_fwspec_get(device->dev);
       ^
drivers/gpu/drm/nouveau/nvkm/subdev/ltc/gp10b.c:39:17: error: struct iommu_fwspec has no member named ids
   u32 sid = spec->ids[0] & 0xffff;

Seletc IOMMU_API under config DRM_NOUVEAU to fix this.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/Kconfig

@@ -2,6 +2,7 @@
 config DRM_NOUVEAU
 	tristate "Nouveau (NVIDIA) cards"
 	depends on DRM && PCI && MMU
+	select IOMMU_API
 	select FW_LOADER
 	select DRM_KMS_HELPER
 	select DRM_TTM