drm/vblank: Add FIXME comments about moving the vblank ts hooks

This is going to be a bit too much, but good to have at least a small
note about where this should all head towards.

Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170509140329.24114-3-daniel.vetter@ffwll.ch