drm/atomic: Reject FLIP_ASYNC unconditionally

drm/atomic: Reject FLIP_ASYNC unconditionally

It's never been wired up. Only userspace that tried to use it (and
didn't actually check whether anything works, but hey it builds) is
the -modesetting atomic implementation. And we just shut that up.

If there's anyone else then we need to silently accept this flag no
matter what, and find a new one. Because once a flag is tainted, it's
lost.

Reviewed-by: Maarten Lankhorst <[email protected]>
Reviewed-by: Nicholas Kazlauskas <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Michel Dänzer <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Adam Jackson <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: David Airlie <[email protected]>
Cc: [email protected]
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]