msm: mdss: update initial line buffering for DSC

Change initial line buffering calculations for DSC as per updated
recommendations. Without enough buffering, in certain conditions DSI
underflow can be observed.

CRs-fixed: 932124
Change-Id: Ia1b04558104061020981ade9a9c86777902360c2
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>