msm: mdss: Adding source histogram overlay API

Allow "overlay set" IOCTLs to enable and disable the histograms on the
source side using this struct and flag. Allows for the first frame of the
overlay request to be captured by histogram.

Change-Id: I36c84acc5d5b5e57e59ca7e8ae43c07708cf4783
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>