drm/bridge: lt8912b: Manually disable HPD only if it was enabled

lt8912b only calls drm_bridge_hpd_enable() if it creates a connector and
the next bridge has DRM_BRIDGE_OP_HPD set. However, when calling
drm_bridge_hpd_disable() it misses checking if a connector was created,
calling drm_bridge_hpd_disable() even if HPD was never enabled. I don't
see any issues caused by this wrong call, though.

Add the check to avoid wrongly calling drm_bridge_hpd_disable().

Fixes: 3b0a01a6 ("drm/bridge: lt8912b: Add hot plug detection")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230804-lt8912b-v1-3-c542692c6a2f@ideasonboard.com