drm/msm/mdp5: Add plane blending operation support for MDP5 (v2)

drm/msm/mdp5: Add plane blending operation support for MDP5 (v2)

This change is to add properties alpha/zpos/blend_mode to mdp5 plane
for alpha blending operation to generate the blended output.
v1: Initial change
v2: Change "premultilied" property to enum (Rob's comment)

Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>
[Don't actually expose alpha/premultiplied props to userspace yet
pending a chance for discussion and some userspace to exercise it]
Signed-off-by: Rob Clark <robdclark@gmail.com>