msm: mdss: reduce minimum mdp ib bandwidth request

The minimum mdp ib for usecases such as portrait video is high,
causing unnecessary power increase, so it is reduced to a optimal
value for the purpose of both power saving and reducing chances of
underruns.

CRs-fixed: 538989
Change-Id: Ibabf16ef3081ef85d181cc8a70e4cfbe029156c1
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>