drm/msm/sde: add validation checks for source split

Planes configured on the same blend stage must be
contiguous by their destination width, same destination
height and yoffsets. This changes validates the source
split pair before staging them.

Change-Id: I6e77b48a29477c4594e4693d1be95f91b73f1ed2
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>