drm/fsl-du: Implement some semblance of vblank event handling

No idea how exactly fsl-du commits hw state changes, but here in flush
is probably the safest place.

While at it nuke the dummy functions.

v2: Be more robust and either arm, when the CRTC is on, or just send
the event out right away.

Cc: Stefan Agner <stefan@agner.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-4-git-send-email-daniel.vetter@ffwll.ch