drm/rockchip: cdn-dp-core: add missing clk_disable_unprepare() on error in cdn_dp_grf_write()

After calling clk_prepare_enable(), clk_disable_unprepare() need
be called when calling regmap_write() failed.

Fixes: 1a0f7ed3

 ("drm/rockchip: cdn-dp: add cdn DP support for rk3399")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210519134928.2696617-1-yangyingliang@huawei.com