drm/msm/mdp5: Enable 3D mux in mdp5_ctl

3D mux is a small block placed after the DSPPs in MDP5. It can merge
2 LM/DSPP outputs and feed it to a single interface.

Enable 3D Mux if our mdp5_pipeline has 2 active LMs. This check
will need to be made more specific later when we add Dual DSI
support with source split enabled. In that use case, each LM feeds to a
separae INTF, so the 3D mux isn't needed.

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