drm/bridge: it6505: Add pre_enable/post_disable callback

Add atomic_pre_enable and atomic_post_disable callback to make sure the
bridge is not powered off until atomic_post_disable is called. This
prevents a power leakage when it6505 is powered off, but the upstream
DRM bridge is still sending display signals.

Fixes: b5c84a9e ("drm/bridge: add it6505 driver")
Signed-off-by: Pin-yen Lin <treapking@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221004044943.2407781-3-treapking@chromium.org