drm/msm/sde: adjust dp timings for widebus configuration

DP horizontal active, porch and total timing configurations
need to divide by 2 when widebus mode is on. This change
corrects the timings accordingly.

Change-Id: Ic739d147fcb893a04d7b08819d99b1f349b4a24c
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>