msm: mdss: Fix check for highest of overlap and prefill bw calc

This check is supposed to be used to get the max between overlap and
prefill bandwidth value. This value is used when calculating the ib
bandwidth value to request when bus bandwidth is updated. Having a value
that is too low results in mdp underrun.

Change-Id: Ie1302737f004d8e030f8d61c75d4301fe99b1069
Signed-off-by: Terence Hampson <thampson@codeaurora.org>