msm: mdss: update latency buffer bytes calculation for prefill bandwidth

This is a s/w change to reflect the new h/w update of latency buffer
byte calculation for prefill bandwidth. Previously this number of
latency buffer bytes is the amount of allocated smp bytes, however,
for some use cases this doesn't not match the bandwidth equation
result. So for the purpose of matching mdp prefill bandwidth
equations, a standalone calculation is needed.

Change-Id: Ic74599d2c2bfe9610ebc22dee137a0df7171dcb5
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>