drm/nouveau/platform: fix probe error path

A "return 0" found its way in the middle of the error path of
nouveau_platform_probe(), remove it as it will make the kernel crash if
we try to unload the module afterwards.

While we are at it, also remove the IOMMU domain if it has been created,
as we should.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>