msm: mdss: update check bw check for ab/ib

Update the bandwidth check for those cases where only
ab changes and ib is greater than ab. The current
logic takes the max of both and since ib is max, the
vote will not kick-in. This change is intended to
solve this problem.

Change-Id: Icbd6d9aa5221784bc1c88ae1965c944503ac78dc
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>