drm: panel: simple: Allow specifying the delay from prepare to enable

On the panel I'm looking at, there's an 80 ms minimum time between HPD
being asserted by the panel and setting the backlight enable GPIO.
While we could just add an 80 ms "enable" delay, this is not ideal.
Link training is allowed to happen in parallel with this delay so the
fixed 80 ms delay over-delays.

We'll support this by logging the time at the end of prepare and then
delaying in enable if enough time hasn't passed.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201109170018.v4.3.Ib9ce3c6482f464bf594161581521ced46bbd54ed@changeid