drm/dp: Add drm_dp_channel_coding_supported() helper

Add a helper to check whether the sink supports ANSI 8B/10B channel
coding capability as specified in ANSI X3.230-1994, clause 11.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021143437.1477719-5-thierry.reding@gmail.com