video: msm: Always queue histogram work & lock histogram calls

Always queue histogram read worker in order to gain concurrency protection
from the histogram management mutex (move previous checks/calls into worker
itself).

Lock the mdp_histogram_start and mdp_histogram_stop calls with the same
lock used currently by mdp_do_histogram to maintain that every do_histogram
called between a start and stop will be completed.

Add a timeout to do_histogram wait_for_completion so that it will unlock
and yield to other histogram calls if it waits for longer than 33 msec.

CRs-fixed: 363763
Change-Id: Icc503aed4e60dd01941682003ca3dbc844aed8d9
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>