They only complete the page flip events to avoid oops when the drm
file closes. The core takes care of that now and we can remove this
code.

Cc: Rob Clark <robdclark@gmail.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453756616-28942-8-git-send-email-daniel.vetter@ffwll.ch