drm/rcar-du: use drm_crtc_handle_vblank()

Remove legacy usage of drm_handle_vblank()

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1467677092-5089-5-git-send-email-gustavo@padovan.org