slimport: anx7808: fix incorrect edid reading request

Previous value might cause that edid specific command
(reset DDC channel of MyDP RX) was not sent correctly.
And then screen blinking issue happened on HDCP failure.

This fixes this issues.

Change-Id: I03c32e72b3c8fabcbfe5476357c5feacbaa61bf0
Signed-off-by: Devin Kim <dojip.kim@lge.com>