drm/bridge: ti-sn65dsi86: Use pm_runtime autosuspend

Let's make the bridge use autosuspend with a 500ms delay. This is in
preparation for promoting DP AUX transfers to their own sub-driver so
that we're not constantly powering up and down the device as we
transfer all the chunks.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210423095743.v5.11.I4c0b4a87e4dc19e5023b4d0a21bbfa6d9c09ebd8@changeid