drm/panel-simple: Non-eDP panels don't need "HPD" handling

All of the "HPD" handling added to panel-simple recently was for eDP
panels. Remove it from panel-simple now that panel-edp handles eDP
panels. The "prepare_to_enable" delay only makes sense in the context
of HPD, so remove it too. No non-eDP panels used it anyway.

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.v5.9.I77d7a48df0a6585ef2cc2ff140fbe8f236a9a9f7@changeid