drm/nouveau: sgdma: add comment around suspiscious error handler

Common programming sense dictates that resources allocated by a function
are freed by this function should it fails, but this is not the case for
the allocated structure of nouveau_sgdma_create_ttm(). It seems that
n00b contributors attempt to fix this one like bugs flying towards a bug
zapper, so add a comment to hopefully prevent this from happening
anymore.

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