drm/bridge_connector: rely on drm_kms_helper_poll_* for HPD enablement

Use drm_connector's helpers enable_hpd and disable_hpd to enable and
disable HPD automatically by the means of drm_kms_helper_poll_*
functions. As the drm_bridge_connector_enable_hpd() and
drm_bridge_connector_disable_hpd() functions are now unused, replace
them with stubs to ease driver migration.

Enabling the HPD from drm_bridge_connector_init() can happen too early,
before the driver is prepared to handle HPD events. As the
drm_bridge_connector_enable_hpd() is empty anyway, drop this call
anyway.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221102180705.459294-4-dmitry.baryshkov@linaro.org