drm/nouveau/nouveau: fix incorrect sizeof on args.src an args.dst

drm/nouveau/nouveau: fix incorrect sizeof on args.src an args.dst

The sizeof is currently on args.src and args.dst and should be on
*args.src and *args.dst. Fortunately these sizes just so happen
to be the same size so it worked, however, this should be fixed
and it also cleans up static analysis warnings

Addresses-Coverity: ("sizeof not portable")
Fixes: 12bcf5c1d7a4 ("nouveau: simplify nouveau_dmem_migrate_vma")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>