msm: mdss: Fix dual-DSI mixer swap

In dual-DSI panel configuration it is possible that panels are physically
swapped on the device. In those situations mixer swap is done. There is
an old dead code in current implementation which leads to failures in
mixer swap use-case. Fix these failures by removing this dead code.

Change-Id: Ib0d7141e322f18bcf1060bc5ab2a404dc41a4cc2
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>