msm: mdss: add DSI control parameters to debugfs

Export DSI control properties in debugfs nodes. DSI commands
and properties can be get and set by reading and writing
debugfs nodes which helps debug and panel bring up instead
of changing dtsi files.

Change-Id: I768a85447d88167894c46eb0770d2644910f84cd
Signed-off-by: Xu Yang <yangxu@codeaurora.org>