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

drivers/gpu/drm/drm_atomic_uapi.c

···
 	if (arg->reserved)
 		return -EINVAL;
 
-	if ((arg->flags & DRM_MODE_PAGE_FLIP_ASYNC) &&
-			!dev->mode_config.async_page_flip)
+	if (arg->flags & DRM_MODE_PAGE_FLIP_ASYNC)
 		return -EINVAL;
 
 	/* can't test and expect an event at the same time. */