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: Iddf151eed53c8ae73418741b2775ee1dd0ef450c
Signed-off-by: Terence Hampson <thampson@codeaurora.org>
Signed-off-by: Vinu Deokaran <vinud@codeaurora.org>
Linaro gcc toolchain