drm/nouveau/nouveau: fix the start/end range for migration

The user level OpenCL code shouldn't have to align start and end
addresses to a page boundary. That is better handled in the nouveau
driver. The npages field is also redundant since it can be computed
from the start and end addresses.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>