drm/panel-simple: Power the panel when probing DP AUX backlight

When I tried booting up a device that needed the DP AUX backlight, I
found an error in the logs:
  panel-simple-dp-aux: probe of aux-ti_sn65dsi86.aux.0 failed with error -110

The aux transfers were failing because the panel wasn't powered. Just
like when reading the EDID we need to power the panel when trying to
talk to it. Add the needed pm_runtime calls.

After I do this I can successfully probe the panel and adjust the
backlight on my board.

Fixes: bfd45140 ("drm/panel-simple: Support DP AUX backlight")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714093334.1.Idb41f87e5abae4aee0705db7458b0097fc50e7ab@changeid