video: msm: Check for multiple readers of histogram

Check to ensure that only one reader attempts to read one frame's worth of
data from a histogram block. Prevent the case where two readers attempt to
get data, but end up causing a loop where neither of them do.

CRs-Fixed: 361640
Change-Id: I48878d9fe02d96f306894ed9acc47bf41cd9984b
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>