drm/panel: simple: Support panels with HPD where HPD isn't connected

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 support in simple-panel for this concept.

At the moment we will co-opt the existing "prepare" delay to keep
track of the delay and we'll use a boolean to specify that a given
panel should only apply the delay if the "no-hpd" property was
specified.

Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181025222134.174583-2-dianders@chromium.org