drm/msm/dsi-staging : Update DSI PLL delay calculation

This is a minor correction on top of previous change
done through commit ff50b197 ("disp: msm: dsi:
Update pll delay calculation as per latest DSI HPG").

Change-Id: I7d8844e8430a58bdc09f0a26ee761f5ed23b8a15
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>