drm/msm/dp: fix mode switch issue with fec on

Switching mode may trigger link-training as part of ctrl_on.
Driver needs to turn off fec on tx side before performing the
link-training if previous link configuration is enabled with
fec. Performing link-training without turning off fec causes
link training failures.

The change ensures to disable fec configuration before
performing the link-training.

CRs-Fixed: 2408160
Change-Id: I122d07a8e5bd70679430c3cfab2739b5f98af872
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>