drm/panel: simplify the return expression of td028ttec1_prepare

Simplify the return expression.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200921131018.91513-1-miaoqinglang@huawei.com