msm: mdss: Fix source split validation

The source split detection logic looks at only consecutive
layers in the layer list. This change looks at all previous
layers in the list instead of just the previous one.

Change-Id: I8b135a10ef1e5062d87c0784515a9eeb6ea4a450
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>