drm: Add ddc link in sysfs created by drm_connector

drm: Add ddc link in sysfs created by drm_connector

Add generic code which creates symbolic links in sysfs, pointing to ddc
interface used by a particular video output. For example:

ls -l /sys/class/drm/card0-HDMI-A-1/ddc
lrwxrwxrwx 1 root root 0 Jun 24 10:42 /sys/class/drm/card0-HDMI-A-1/ddc \
	-> ../../../../soc/13880000.i2c/i2c-2

This makes it easy for user to associate a display with its ddc adapter
and use e.g. ddcutil to control the chosen monitor.

This patch adds an i2c_adapter pointer to struct drm_connector. Particular
drivers can then use it instead of using their own private instance. If a
connector contains a ddc, then create a symbolic link in sysfs.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d470def6cd661b777faeee67b5838a4623c4010e.1564161140.git.andrzej.p@collabora.com