msm: mdss: Cleanup the interface given by mdss/mdp

Change irq interface provided by mdp from exporting symbols to
function pointers. This will help in making mdp3 work with dsi 6G.

Change-Id: I07a2acf56b75a2a6d83c0522d38efe3754bb8765
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>