Claude review: drm/v3d: Reject invalid out_sync handles in submit ioctls

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Mirrors patch 6's treatment of in_sync for the out_sync path. Previously, a non-zero invalid `out_sync` handle was silently ignored. Now, `drm_syncobj_find()` is done up-front and returns `-ENOENT` on failure.

`v3d_put_multisync_post_deps()` is generalized into `v3d_submit_put_post_deps()` which handles both the single-sync case (`sync_out`) and multi-sync case (`se->out_syncs`).

**Same UAPI note as patch 6**: Userspace passing invalid non-zero `out_sync` handles will now get `-ENOENT` instead of silent success. Correct behavior, potential compat concern if buggy userspace exists.

**Minor observation**: The early return paths for `sync_out` lookup failure happen before any job allocation, so there's no cleanup needed — this is correct:
```c
if (args->out_sync && !(se.flags & DRM_V3D_EXT_ID_MULTI_SYNC)) {
    sync_out = drm_syncobj_find(file_priv, args->out_sync);
    if (!sync_out)
        return -ENOENT;
}
```

No issues.

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