drm/msm/mdp5: Always generate active-high sync signals for DSI

DSI video mode engine can only take active-high sync signals. This
change prevents MDP5 sending active-low sync signals to DSI in any
case.

Signed-off-by: Hai Li <hali@codeaurora.org>
Tested-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>