Revert "drm/msm/sde: modify DSC initial_lines calculation"

This reverts commit 3f3286e8.
Retain the older calculation for the DSC initial_lines
parameter.

Change-Id: I125fbe6a21d946dddbc13c2b30a0ec564151c62c
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>