msm: mdss: configure panel split based on configuration

Current code assumes when Dual DSI configuration is used that width will
be twice the left panel resolution. This may not always be the case and
should instead look at the configuration to get this information.

Change-Id: I5c2ac608f46d7c0ce1acbffe002d69fca44afaf7
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>