msm: mdss: update bandwidth equation when pingpong split enabled

When pingpong split is enabled, single DSPP drives the two DSI
panels and bandwidth needs to be adjusted based on the complete
panel width. In the existing implementation, only left half of the
panel is being considered for bandwidth vote. This change addresses
issue with bandwidth calculations when pingpong split is enabled.

Change-Id: I24ab9dee80c75f25f8c41501f6b635d98de512a2
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>