msm: mdss: update mdp clock rate with pingpong split enabled

When pingpong split is enabled, single DSPP output is driving two
interfaces and MDP clock has to be 2x times the pixel clock. In the
current implementation, pingpong split is not considered in the MDP
clock calculations. This change accounts for pingpong split and update
MDP clock accordingly.

Change-Id: I85a86fa747c908f76bd01faf36fe310d502e2121
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>