drm/rockchip: Set line flag config register in vop_crtc_enable

We need to set vop config done after update line flag config, it's a
new requirement for chips newer than rk3368.

Since we would only use line flag irq for vact_end, let's move it to
vop_crtc_enable.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Acked-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1493276057-4516-1-git-send-email-jeffy.chen@rock-chips.com