msm: mdss: add support to register secondary fb on DSI device

In the current implementation, DSI0/DSI1 device is always mapped
to frame buffer-0. For dual DSI configuration, we need to
register both the DSI devices to different frame buffers. Add driver
and dtsi support to register DSI device on primary/secondary
frame buffer at runtime based on the DSI configuration.

Change-Id: Iac872723711c5d0264088c4f3b53d1385fd9ffe0
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>
Signed-off-by: Vinu Deokaran <vinud@codeaurora.org>
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>