msm: mdss: Fix AD configuration for single DSI case

The default configuration for AD config_buffer_mode register is correct
for dual DSI case, but not for single DSI case. This change correctly
set the AD config_buffer_mode for single DSI case.

Change-Id: I8b1b665e027e925d607fda078cc453a5406f85ea
Signed-off-by: Ping Li <pingli@codeaurora.org>