mdss: mdp: ad calc worker cleanup

Assertive display block can be configured in auto trigger mode,
which removes the requirement of issuing a start calculation
command. Change updates the ad calc worker to remove the kick off
of AD for each vsync and auto trigger mode is made the default mode.

Change-Id: Ie898a595cc9d3c8e521fffc8b3cde240dcbdce0e
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>