mdp: flo: Set minimum IB value for 4 layers

For 4 layer MDP composition, if the total composed area
doesn't cover the whole screen, there is a potential hole
in our calculation where we don't request enough bandwidth
especially for larger displays such as flo.
Pin the minimum bandwidth to ~1.6G (empirical) to fix this.

Bug: 10954796
Change-Id: I7a3de0ac82326271202833d5e79d851d871eeabf
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>