msm: mdss: add support to switch the DSI clock dynamically

Add change to support changing the DSI bit clock dynamically
for video mode panels. This helps to avoid interference of
DSI clock with other subsystems runtime.

Change-Id: I05790a6dd9d8a2fc3cf31727d032e5220d6164e5
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>