drm/msm/dsi-staging: fix default value for DSI clockout control

DSI clockout control setting for t-clk-pre and t-clk-post is an
optional parameter for certain chip versions. When these bindings
are not specified, current implementation has default values which
may result in incorrect settings. Fix this by setting the default
value to 0x00.

Change-Id: Iecbc4f2d6311263c7dac4af40f72cf746e48cb86
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>