msm: mdss: dsi: Add ULPS mode support for dual dsi configuration

For dual dsi panels with broadcast mode enabled, the two dsi
controllers work in a master-slave configuration. Add support for
enabling ultra low power state (ULPS) mode in this use case.

Change-Id: I9e4383ca60174d6ffa599ce6caa47676a5eea216
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>