drm/msm/dsi-staging: fix few DSC parameter calculations

Fix the calculation for few DSC parameters and update
few DSC related constant values according to the systems
team recommendation.

Change-Id: Idc5f18ac73cd6f81a53a635de8a5dd3923a7c383
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>