msm: display: update histogram interface for MDP5

MDP5 supports 256 histogram bins now, which is not able to be
stored in the 8 bit unsigned integer used currently. Increase
this variable to be able to accommodate it.

Change-Id: Ie62ba14cb51f3db9ab3e0b7ad94ac59827611210
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>