drm/rockchip: Nuke pending event handling in preclose

This is now handled by the core, drivers can totally ignore lifetime
issues of drm events.

Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Mark yao <mark.yao@rock-chips.com>
Tested-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-11-git-send-email-daniel.vetter@ffwll.ch