msm: mdss: fix quality issues when source split is disabled

When source split is disabled on split display panels, a layer can be
split un-evenly into two pipes. When in this condition along with
vertical decimation specified by user-space, it is possible that driver
infuse extra decimation if per pipe BW is exceeded. Now when we do this,
it is possible that both halves of the same layer get different decimation
and can lead to quality difference between two panels. Fix this by not
allowing extra decimation when split display is enabled.

Change-Id: Ib47953fd94400b01d02ee07059bf008310abbc84
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>