drm/panel: p079zca: Add variable unprepare_delay properties

When panel power down, p079zca need delay between reset and disable
power supply, but p097pfg does not need it. Similarly p097zca needs
a delay after entering panel sleep mode. So add two delay properties,
so we can meet these two panel power down sequence.

Signed-off-by: Lin Huang <hl@rock-chips.com>
[add sleep-mode delay]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180702102721.3546-3-heiko@sntech.de