drm/msm/dp: update mode validation logic for widebus

Update the mode validation logic by halving the mode clock when
widebus is enabled.

Change-Id: I8f060d8b60403aa5020496983bec0b3e2878b08b
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>