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: b5c84a9edcd4 ("drm/bridge: add it6505 driver")
Signed-off-by: Pin-yen Lin <treapking@xxxxxxxxxxxx>