drm/nouveau/svm: fix migrate page regression

The patch to add zero page migration to GPU memory inadvertently included
part of a future change which broke normal page migration to GPU memory
by copying too much data and corrupting GPU memory.
Fix this by only copying one page instead of a byte count.

Fixes: 9d4296a7

 ("drm/nouveau/nouveau/hmm: fix migrate zero page to GPU")
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>