libplacebo/display: prevent ICC 3DLUT cache collisions

Since these 3DLUTs may be persistently cached, we run into the risk of cache collisions if the profile signature does not uniquely identify the actual profile used. This invalidates our previous design of simply incrementing the signature on change.

Instead, explicitly compute this, and cache the result until the next time UpdateIccProfile gets called.

Fixes: db063d7f