msm: mdss: Add get pan cfg and get iommu dom to common interface

Add get panel configuration and get iommu domain functions to
common utility interface. This will make sure that DSI 6G gets
the right information on ferrum which has mdp3.

Change-Id: Ib5a9d70b8202eaa9e3003475398ecef88dfb573d
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>