msm: mdp3: prevent NOC error cases with histogram during ctl off

During ctl off there can be a race condition of histogram start
call of ioctl and histogram stop call of ctl off sequence. If
histogram start were to come after histogram stop of ctl off
sequence and set histo status, there would be a delayed hist
stop ioctl that will cause NOC error. Histogram start itself
might cause NOC error if footswitch control came in just before
the start call. So to prevent these histogram start and ctl off
sequence are made exclusive using the session lock.

Change-Id: If9a42bd52b0df0537ca5cc3fd6d346f7b7b4b185
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>