msm: mdss: Sum split-display histograms into left histogram data

For every histogram collect in a split display configuration, a
concatenation scratch array was alloced in order to sum the histograms
of the multiple DSPPs. This change improves performance by removing the
allocation and summing the histograms into the histogram array of the
first DSPP in the split display.

Change-Id: Id07cdfc2175bdce0f3b662daa70a014986f73eb4
Signed-off-by: Benet Clark <benetc@codeaurora.org>