WIP: drm/virtio: Support fence-passing feature

Support extended version of VIRTIO_GPU_CMD_SUBMIT_3D command that allows
passing in-fence IDs to host for waiting, removing need to do expensive
host-guest roundtrips in a case of waiting for fences on a guest side.

Guest userspace must enable new VIRTGPU_CONTEXT_PARAM_FENCE_PASSING flag
and host must support new VIRTIO_GPU_F_FENCE_PASSING feature in order to
activate the fence passing for a given virtio-gpu context. Array of
in-fence IDs is then prepended to the VIRTIO_GPU_CMD_SUBMIT_3D's data,
the previously unused padding field of the command is reused for the
number of in-fences.

A new VIRTGPU_EXECBUF_SHARED_FENCE flag is added to the job submission
UAPI and must be set by userspace if it wants to make fence shareable
with/on host. Certain jobs won't want to share fence, in particular Venus
will benefit from this flag.

Link: virgl/virglrenderer!1138
Link: https://gitlab.freedesktop.org/digetx/qemu/-/commits/native-context-iris
Link: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4679609


Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>