drm/sun4i: Implement some semblance of vblank event handling

atomic_flush seems to be the right place, right after we commit the
plane updates. Again use the fullproof version, since the pipe might
be off.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
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/1465388359-8070-6-git-send-email-daniel.vetter@ffwll.ch