Some eDP panels that are designed to be always connected to a board
use their HPD signal to signal that they've finished powering on and
they're ready to be talked to.

However, for various reasons it's possible that the HPD signal from
the panel isn't actually hooked up.  In the case where the HPD isn't
hooked up you can look at the timing diagram on the panel datasheet
and insert a delay for the maximum amount of time that the HPD might
take to come up.

Let's add a property in the device tree for this concept.
Reviewed-by: Sean Paul <sean@poorly.run>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181025222134.174583-1-dianders@chromium.org