msm: mdss: Relocate Assertive Display register access

Move the register writes for Assertive Display (AD) into the dspp_setup
function. This separates the state of AD from the configuration of the
hardware and adds to the implementation of dual pipe AD feature.

Change-Id: I9f21f9461b3d4ae3c29ad52e17ad33d8e6e4b904
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>