msm_fb: display: add scaling factor for mdp ab

system profiling found that mdp downscaling needs larger bandwidth
that is related to mdp ab request, so adding scaling factor to mdp ab
calculation reduces the chance of underruns.

ab - scaling factors - port0 and port1 split
ib - 1.5 x ab - no split and same on both ports

Change-Id: I4fde87fce1cd292e317a8261e4700cf98662d09d
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>