drm/omap: Don't call HPD registration operations recursively

Instead of calling the hot-plug detection callback registration
operations (.register_hpd_cb() and .unregister_hpd_cb()) recursively
from the display device back to the first device that provides hot plug
detection support, iterate over the devices manually in the DRM
connector code. This moves the complexity to a single central location
and simplifies the logic in omap_dss_device drivers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>