drm/msm/mdp5: don't pre-reserve LM's if no dual-dsi

If there is only a single DSI interface, don't reserve the first two
layer-mixers for the dual-DSI use-case.

This was causing problems for WB, not being able to assign a LM, on
8x16, which has only two LM's and a single DSI.

Signed-off-by: Rob Clark <robdclark@gmail.com>