linux-explicit-synchronization: Warn about using the protocol while using graphics APIs

Graphics APIs are expected to use this protocol under the hood, and
since there can only be one user of explicit synchronization per
surface, warn about using the protocol directly in such cases.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>