msm: mdss: Verify histogram size before sending to user

Add check to DSPP histograms to verify that the number of pixels collected
is the same as the resolution of the region being processed.

Change-Id: I21acd1b4a25d06391c853e7495ca3fb3bb0ffef4
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>