video: msm: Dynamically toggle histogram interrupt

Make histogram enable/disable functions also enable/disable the histogram
"interrupt enabled" bit on the MDP enabled interrupts register.

CRs-Fixed: 398556
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>
Change-Id: I56f811e88cf3fd44ae0a2df8a6e5de32ba9a0c1c