msm_fb: display: keep a minimum mdp ib bandwidth request

The calculated ab and ib bandwidth requests are very small which cause
underruns if mdp composites only very small layers. For these use
cases keeping a minimum ib request based on the baselayer size can
prevent underruns and no power impact is observed.

CRs-fixed: 472444
Change-Id: I08e77610aafdfa84c5c2a882a0c95a51950aa069
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>