drm/msm/dpu: initial support for merge3D hardware block

Add initial support for merge3D hardware block on SM8[12]50. Merge3D is
reposible for merging contents of two LMs (two PPs) into single
interface.

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