drm: add support for DisplayPort CEC-Tunneling-over-AUX

This adds support for the DisplayPort CEC-Tunneling-over-AUX
feature that is part of the DisplayPort 1.3 standard.

Unfortunately, not all DisplayPort/USB-C to HDMI adapters with a
chip that has this capability actually hook up the CEC pin, so
even though a CEC device is created, it may not actually work.

Change-Id: I8dc417c30ee387cbbd4fd29059c79e9457896291
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180711132909.25409-2-hverkuil@xs4all.nl
Git-commit: 2c6d1fffa1d9b0a5b5ac1a23be9ad64abe60910d
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[vvalluru@codeaurora.org: resolved merge conflicts]
Signed-off-by: Venkata Prahlad Valluru <vvalluru@codeaurora.org>