drm/msm/sde: update src split check to allow two sspp on each z_order

Current source split validation check does not allow
two source pipes only on left or only on right. This is
supported on HW but SW was keeping restriction due to
unknown reason. This change keeps source split validation
in sync with MDSS HW design.

Change-Id: I9a0eec739dc398e5fa7f59e8347290e0b014caf1
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>