msm: mdss: remove duplicated aquiring of cmd_mutex

It is incorrect to acquire the same mutex twice in case of
ESD read check because this mutex will be acquired in nested
function, so remove it in order to avoid dead lock.

Change-Id: I48c18195e0ac30b271bfc041a99547049ee6b04a
Signed-off-by: Ray Zhang <rayz@codeaurora.org>