video: msm: Ensure idle state before enabling histogram

Disable interrupt, set state flags to false, cancel histogram collection,
and clear the workqueue before attempting to enable the histogram. This
guarantees that this is the only configuration of the histogram running.

Change-Id: I222932db40494724b3efdb2e5bea08a849095c1e
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>