drm/nouveau: chan: provide nouveau_channel_kill()

The new VM_BIND UAPI implementation introduced in subsequent commits
will allow asynchronous jobs processing push buffers and emitting fences.

If a job times out, we need a way to recover from this situation. For
now, simply kill the channel to unblock all hung up jobs and signal
userspace that the device is dead on the next EXEC or VM_BIND ioctl.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230804182406.5222-10-dakr@redhat.com