drm/msm/dsi-staging: implement dynamic DSI clock

Dynamic DSI clock setting is implemented by re-calculating
DSI link clocks from DSI bitrate in sysfs interface and
re-enaling the clock with new bitrate while there is no
DSI data transferring.
Currently, the feature is only supported in DSI command
mode.

CRs-Fixed: 2177917
Change-Id: I1192e1980726ad632b619e0fe13034f79c169afa
Signed-off-by: Yujun Zhang <yujunzhang@codeaurora.org>