drm/bridge: ti-sn65dsi86: fetch bpc using drm_atomic_state

Rather than reading the pdata->connector directly, fetch the connector
using drm_atomic_state. This allows us to make pdata->connector optional
(and thus supporting DRM_BRIDGE_ATTACH_NO_CONNECTOR).

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: Steev Klimaszewski <steev@kali.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220711092117.360797-2-dmitry.baryshkov@linaro.org