drm/sun4i: crtc: Call drm_crtc_vblank_on / drm_crtc_vblank_off

Make sure that the CRTC code will call the enable/disable_vblank hooks.

Otherwise, since the refcounting will be off, we might end up in a
situation where the vblank management functions are called while the CRTC
is off.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221125703.4595-3-maxime.ripard@bootlin.com