msm: mdss: set proper primary panel flag for dual DSI case

In case of dual DSI configuration if any of the DSI
controller is using bridge chip(HDMI) then consider that
as secondary display or else consider DSI1 controller as
secondary always. This change takes care of this assumption.

Change-Id: Ia24c45de2af41144e76b102232b7481ca5c5acf2
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>
Signed-off-by: Vinu Deokaran <vinud@codeaurora.org>