drm/nouveau: use ttm_bo_reserve_slowpath in validate_init, v2

Similar rationale to the identical commit in drm/ttm.
Instead of only waiting for unreservation, we make sure we actually
own the reservation, then retry to get the rest.

Changes since v1:
 - Increase the seqno before calling ttm_bo_reserve_slowpath

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>