msm: mdss: Fix panel orientation handling for split displays

For split display panels, when panel has 180 flip or HFLIP,
the source layers needs to be swapped instead of the mixers.
SDM will take care of the source swap and rectangle
recalculations.
This change is to remove the mixer swap, and to expose the panel
flip property through sysfs, so that SDM can flip the source
layers to maintain pipe priority.

Change-Id: I47a7dd38dc1e45dd3f7edbd189c9403900b74eea
Signed-off-by: Krishna Chaitanya Devarakonda <kdevarak@codeaurora.org>