msm: mdss: add fudge factor for command mode panel

In case of command mode, we have to take account of
bus inefficiency, otherwise some performance drops
could be hit. Set 2x ratio for 8939 and keep 1x
ratio as default for other targets.

CRs-fixed: 719913
Signed-off-by: Ray Zhang <rayz@codeaurora.org>
Change-Id: I5dd5e69172e743b928182220a9588de9f454ca4e