drm/nouveau: fix missing return statement in nouveau_ttm_tt_unpopulate

nouveau_ttm_tt_unpopulate() is supposed to return right after calling
ttm_dma_unpopulate() in the case of a coherent buffer. The return
statement was omitted, leading to the pages being unmapped twice. Fix
this.

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