drm/omap: Switch the HDMI and VENC outputs to drm_bridge

The TPD12S015, OPA362 and analog and HDMI connectors are now supported
by DRM bridge drivers, and the omapdrm HDMI and VENC outputs can be
handled through the drm_bridge API. Switch the outputs to drm_bridge by
making the next bridge mandatory and removing the related
omapdrm-specific display drivers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-40-laurent.pinchart@ideasonboard.com