disp: msm: sde: fix video mode prefill lines for RSCC

The RSCC static wakeup and the bandwidth trigger for
the downvotes are based on the prefill lines. Reduce
the prefill lines based on the panel vertical front
porch to avoid issuing bw downvotes during active
region of the previous frame.

Change-Id: I408209ba308c32e71d9f70c5ed7e60c134877c84
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>