drm/hisilicon: Implement some semblance of vblank event handling

atomic_flush seems to be the right place, but I'm not entirely sure
whether this will catch them all. It could be that when disabling the
crtc we'll miss the vblank.

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: Xinliang Liu <xinliang.liu@linaro.org>
Cc: Xinwei Kong <kong.kongxinwei@hisilicon.com>
Cc: Archit Taneja <architt@codeaurora.org>
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/1465388359-8070-5-git-send-email-daniel.vetter@ffwll.ch