mdss: display: add edp aux channel support toward link training

Edp aux channel is the communication channel between host and
client (panel). Host reads edid back from panel and issues dpcd
command to panel through aux channel. This patch adds aux channel
support so that edp link training can be done.

CRs-Fixed: 486135
Change-Id: If29dc8257ecc555296c043f2ebcc6dc5cb400d49
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>