drm/msm/dsi: adjust dsi timing for dual dsi mode

For dual dsi mode, the horizontal timing needs
to be divided by half since both the dsi controllers
will be driving this panel. Adjust the pixel clock and
DSI timing accordingly.

Changes in v3:
- Added Archit's R-b
- Rebase on dsi cleanup set in msm-next

Cc: Sibi Sankar <sibis@codeaurora.org>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>