drm/atomic-helper: Fix reference to drm_crtc_send_vblank_event()

Fix up this reference so that the proper link is generated in the
documentation and so that people don't go chasing after the wrong
function for an embarrassingly long time.

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171012140857.9559-1-thierry.reding@gmail.com