msm: mdss: add support of source split feature

Source split feature relaxes some of the old MDP HW limitations and
enables newer use-cases. This feature is available from MDSS 1.3.0 onwards.

Some of the key highlights are:
* single pipe can be staged on two layer mixers.
* two pipes can be staged at a single blending stage given left pipe
  on that stage is higher priority compared to the right pipe.
* Ex. 1080p video on can be played on a dual-dsi panel using single
       pipe compared to two pipes without this feature, saving power.
* Ex. using above features, two pipes can equally split the load of a high
      downscaling surface and thus reducing peak bandwidth requirements.

Change-Id: Ia08e37aca2ab0e80db6aef6c8e5ef37149b84e3a
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>