drm/msm/sde: stage source pipes on both layer mixer for src_split

The source split requires to stage each source pipe on both
layer mixers. The SDE hardware layer mixer hangs if left only
pipe is not staged on right layer mixer when same stage has
layer mixer crossing source split pipe. This change stages
all the pipes on both layer mixers to avoid layer mixer hang
issue.

Change-Id: Ie5c3361c8cada4e8bafd7a8842a908843d611019
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>