drm/msm/sde: move src_split_order setting to commit phase

Avoid setting the pipe_order flag during the check phase
in the sde pstate as new pstate will not be allocated
if none of the  properties have changed in the plane.
This might lead to the validate from user-thread and
commit from driver-thread to operate on the same pstate
and possibly configuring the wrong pipe_order to the HW.

Change-Id: I69c46c18339fd4dbed9ef5f364563a99e9ee2985
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>