drm_dp_cec: add note about good MegaChips 2900 CEC support

A big problem with DP CEC-Tunneling-over-AUX is that it is tricky
to find adapters with a chipset that supports this AND where the
manufacturer actually connected the HDMI CEC line to the chipset.

Add a mention of the MegaChips 2900 chipset which seems to support
this feature well.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180827075820.41109-3-hverkuil@xs4all.nl

drivers/gpu/drm/drm_dp_cec.c

@@ -16,7 +16,9 @@
  * here. Quite a few active (mini-)DP-to-HDMI or USB-C-to-HDMI adapters
  * have a converter chip that supports CEC-Tunneling-over-AUX (usually the
  * Parade PS176), but they do not wire up the CEC pin, thus making CEC
- * useless.
+ * useless. Note that MegaChips 2900-based adapters appear to have good
+ * support for CEC tunneling. Those adapters that I have tested using
+ * this chipset all have the CEC line connected.
  *
  * Sadly there is no way for this driver to know this. What happens is
  * that a /dev/cecX device is created that is isolated and unable to see