msm: mdss: do not shut down edp link during link training

Use locks to prevent shutting down edp link at panel off if link
training is ongoing, since EDP Link training requires that link
is in running state.

Change-Id: I79715675f0a919815dfc60689215f4771400b28a
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>