msm: mdss: refactor MDSS DSI driver

Current DSI driver implementation treats the two DSI controllers
as separate devices. However, some use cases might require the
two DSI devices to be aware of the global configuration of the
display interfaces. For instance, the panel/board configuration
might require both the DSI controllers to work in tandem and
drive a split DSI panel or else drive two different panels
independently. Add a root DSI node in MDSS dtsi and the two
DSI controller nodes as the child nodes. Refactor the DSI
panel related dtsi files and DSI drivers to support this new
design. The DSI root node contains the global information needed
for the DSI controllers like the DSI h/w configuration needed to
drive the panel(s), the DSI register space ranges etc.

Change-Id: I3661adf9ce2ecbef83ae3267e4aae18e0f0c8d04
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>