drm/panel-edp: Clarify the `prepare_to_enable` description in comments

It's unclear why I originally wrote in the description of
`prepare_to_enable` that "This is not specified in a standard way on
eDP timing diagrams" and then also wrote "It is effectively the time
from HPD going high till you can turn on the backlight." It seems
pretty clear that it's (T4+T5+T6+T8)-min. Either I was confused when I
wrote this or I was looking at some strange panel datasheet that I can
no longer find.

Update the description of the field so it's easier for people to fill
this in. Couch the description with "usually" in case there really was
some weird datasheet where things were specified in a different way.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250521173204.1.Ic0375a9360698592f27afbf1f60f4996d504ed4f@changeid