drm: panel: simple: Defer unprepare delay till next prepare to shorten it

It is believed that all of the current users of the "unprepare" delay
don't actually need to wait the amount of time specified directly in
the unprepare phase.  The purpose of the delay that's specified is to
allow the panel to fully power off so that we don't try to power it
back on before it's managed to full power down.

Let's use this observation to avoid the fixed delay that we currently
have.  Instead of delaying, we'll note the current time when the
unprepare happens.  If someone then tries to prepare the panel later
and not enough time has passed, we'll do the delay before starting the
prepare phase.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201109170018.v4.2.I06a95d83e7fa1bd919c8edd63dacacb5436e495a@changeid