drm/rockchip: Don't continue trying to enable crtc on failure

If vop_enable fails, don't continue on, it causes system hangs.

Reviewed-by: Yakir Yank <ykk@rock-chips.com>
Tested-by: Yakir Yank <ykk@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1471302749-2811-1-git-send-email-seanpaul@chromium.org