msm: mdss: modify how pingpong-split is used for a given panel

In the current implementation, driver makes a decision to use
pingpong-split data path if HW supports pingpong-split and connected
panel is a dual-DSI. However, in the high tier targets like msm8996 it
is possible to use dual-dsi panels in multiple modes, i.e with single
layer mixer + pingpong-split or dual layer mixer. So the correct place to
define if pingpong-split will be used or not is with panel configuration
not with the MDP driver.

In order to support this, MDP device node will continue using its
property qcom,has-pingpong-split to tell driver that underlying HW
supports pingpong-split. Introduce a new panel property,
qcom,split-mode, which will be used to decide if pingpong-split
for a given panel should be used or not.

Change-Id: I210e7f2a8b03d5c8a161241120c7d3c7081fa971
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>