drm/bridge: ti-sn65dsi86: Move drm_panel_unprepare() to post_disable()

We prepared the panel in pre_enable() so we should unprepare it in
post_disable() to match.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210416153909.v4.5.Ia75c9ffe2a2582393a8532d244da86f18b4c9b21@changeid