msm: mdss: reduce mdp prefill bandwidth vote for command mode panel

The current result of mdp prefill bandwidth calculation for command
mode is larger than what is actually needed for many use cases. This
is due to the fact that for command mode panels during prefill time,
there are less buffer prefill requirement or time constraint than that
of video panels. This patch makes adjustment accordingly based on the
updated equations by h/w team.

Change-Id: I06935a29d771202b3f8dc2c62cf948a8ecc81481
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>