drm/panel-edp: Don't re-read the EDID every time we power off the panel

The simple-panel driver is for panels that are not hot-pluggable at
runtime. Let's keep our cached EDID around until driver unload.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210914132020.v5.14.Ib810fb3bebd0bd6763e4609e1a6764d06064081e@changeid