mdp3: Enable sw vsync when dma is not active

Recently in an attempt to enhance sync event reporting we
prevented sw vsync when dma is turned of from toggleing display
off and on.

Change-Id: I71c9be55d36dc92e513d9f5ead4d1e42157285c6
Signed-off-by: Terence Hampson <thampson@codeaurora.org>