drm/bridge_connector: enable HPD by default if supported

Hotplug events reported by bridge drivers over drm_bridge_hpd_notify()
get ignored unless somebody calls drm_bridge_hpd_enable(). When the
connector for the bridge is bridge_connector, such a call is done from
drm_bridge_connector_enable_hpd().

However drm_bridge_connector_enable_hpd() is never called on init paths,
documentation suggests that it is intended for suspend/resume paths.

In result, once encoders are switched to bridge_connector,
bridge-detected HPD stops working.

This patch adds a call to that API on init path.

This fixes HDMI HPD with rcar-du + adv7513 case when adv7513 reports HPD
events via interrupts.

Fixes: c24110a8 ("drm: rcar-du: Use drm_bridge_connector_init() helper")
Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211225063151.2110878-1-nikita.yoush@cogentembedded.com