video: msm: Always queue histogram work

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

CRs-fixed: 363763
Change-Id: If9e3c13b5e4005f81fd1b5d2664af206b1decf5b
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>