[RFC] drm/atomic: Abuse legacy cursor update flag for legacy gamma update too

[Chris Wilson]

Quoting Maarten Lankhorst (2018-02-13 09:12:01)
> Programs like redshift set the legacy gamma for X.org every 5 seconds.
> Because atomic commits wait for vblank completion, we get a frame drop
> every 5 seconds because of the legacy gamma update.
> 
> Work around this by setting the legacy_cursor_update flag, to force
> legacy gamma updates not to be synced against vblank.
> 
> Reported-by: Tholin #intel-gfx
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>

Reminiscing: Remember the time we had all those vblank workers patches,
one of which was to do async gamma updates (apply the last one on the
vblank). Motivated by gnome-shell and the ilk doing a gamma animation
for fade-out on logout (which I guess they gave up on since it happened
to be so slow as to extend logout ;).
-Chris