video: msm: Clear previous histogram data on next startup

Clear the pointer to the userspace histogram struct when histogram is
started. Previously, when the histogram was stopped while a histogram read
was in progress, the pointer was not cleared, causing the next histogram
read to think that someone was already using the histogram.

CRs-Fixed: 361640
Change-Id: If09017149e98b35c49603372c554aa4c97f2de82
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>