drm/bridge: analogix_dp: Ensure the panel is properly prepared/unprepared

Instead of just preparing the panel on bind, actually prepare/unprepare
during modeset/disable. The panel must be prepared in order to read hpd
status and edid, so we need to keep state around the prepares in order
to ensure we don't accidentally turn the panel off at the wrong time.

Reviewed-by: Yakir Yang <ykk@rock-chips.com>
Tested-by: Yakir Yang <ykk@rock-chips.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>