drm/bridge: Add bridge driver for display connectors

Display connectors are modelled in DT as a device node, but have so far
been handled manually in several bridge drivers. This resulted in
duplicate code in several bridge drivers, with slightly different (and
thus confusing) logics.

In order to fix this, implement a bridge driver for display connectors.
The driver centralises logic for the DVI, HDMI, VGAn composite and
S-video connectors and exposes corresponding bridge operations.

This driver in itself doesn't solve the issue completely, changes in
bridge and display controller drivers are needed to make use of the new
connector driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
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/20200226112514.12455-16-laurent.pinchart@ideasonboard.com