drm/nouveau: Add fence fd helpers

Add nouveau_fence_install, which installs a drm fence as a file
descriptor that can be returned to user space.

Add nouveau_fence_sync_fd, which pushes semaphore wait commands for
each Nouveau fence contained within the sync fd.  If the sync fd
contains non-Nouveau fences, those are waited on the CPU.

Add missing fence_value_str and timeline_value_str callbacks to
nouveau fence_ops.

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