msm: mdss: remove extra lock for ctl calls

Some ctl functions need only protection for mixer programming changes,
since all of these calls are made within ov_lock there is no need for
additional protection. Specially since ctl->lock is still held when
mixer configuration is safe to be changed.

Change-Id: I4f39e28f40ef18789a653d02346358b7ca7590c6
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>