drm/panel: auo,b116xw03: fix flash backlight when power on

Delay the backlight on to make sure the video stable.

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200705094514.34526-1-jitao.shi@mediatek.com