drm: when queuing an event with NEXTONMISS, return queued sequence to userspace

If we queue a vblank event but miss it, we should return the actual
sequence number we queued to userspace, so its event handling function
will know which event to look for.

Acked-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>