drm/msm/dpu: enable merge_3d support on sm8150/sm8250

Handle new merge_3d block setup in dpu encoder code. Pass correct mode
and id. Note, that merge_3d blocks are not handled via usual RM
reservation mechanism, as each merge_3d block is tied to two PPs, so by
reserving PP you get merge_3d automatically.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>