msm: mdss: Cleanup handling of dual dsi broadcast mode

When broadcast mode is enabled for dual dsi panels, the two dsi
controllers act in a master-slave configuration. A lot of configuration
in the driver requires special handling in such scenarios. Current
implementation has a lot of redundant code to check for broadcast mode
configurations. Rework the code to remove these and add a unified set of
APIs to query broadcast mode configurations.

Change-Id: I634f94266e8dce28bcaa897ef39e7c3ca9b558e5
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>