drm/panel: simple: Drop prepared_time

This has been superseded by RPM in commit
3235b0f ("drm/panel: panel-simple: Use runtime pm to avoid excessive unprepare / prepare")
and the last user of the non-RPM case has been dropped in commit
b6d5ffc ("drm/panel-simple: Non-eDP panels don't need "HPD" handling")
whatever is left in this driver is just assigned and never used.
Drop the remaining parts.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230709162253.464299-1-marex@denx.de

Author: Marek Vasut

drivers/gpu/drm/panel/panel-simple.c

@@ -141,7 +141,6 @@ struct panel_simple {
 
 	bool prepared;
 
-	ktime_t prepared_time;
 	ktime_t unprepared_time;
 
 	const struct panel_desc *desc;
@@ -351,8 +350,6 @@ static int panel_simple_resume(struct device *dev)
 	if (p->desc->delay.prepare)
 		msleep(p->desc->delay.prepare);
 
-	p->prepared_time = ktime_get_boottime();
-
 	return 0;
 }
 
@@ -566,7 +563,6 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
 		return -ENOMEM;
 
 	panel->enabled = false;
-	panel->prepared_time = 0;
 	panel->desc = desc;
 
 	panel->supply = devm_regulator_get(dev, "power");