It doesn't even show up in any header files and only used iternally.
Originally it was (ab)used to restore the fbcon on lastclose, but that
died with

commit e8e7a2b8
Author: Dave Airlie <airlied@redhat.com>
Date:   Thu Apr 21 22:18:32 2011 +0100

    drm/i915: restore only the mode of this driver on lastclose (v2)
Reviewed-by: NRob Clark <robdclark@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>