drm/nouveau: Support fence fd's at kickoff

Add a new NOUVEAU_GEM_PUSHBUF_2 ioctl that accepts and emits a sync
fence fd from/to user space if the user space requests it by passing
corresponding flags.

Signed-off-by: Lauri Peltonen <lpeltonen@nvidia.com>
drm/nouveau: Support marking buffers for explicit sync

Do not attach fences automatically to buffers that are marked for
explicit synchronization.

Signed-off-by: Lauri Peltonen <lpeltonen@nvidia.com>