drm/msm/mdp5: add support for alpha/blend_mode properties

Hook alpha and pixel blend mode support to be exported as proper DRM
plane properties. This allows using this functionality from the
userspace.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20210525131316.3117809-5-dmitry.baryshkov@linaro.org


Signed-off-by: Rob Clark <robdclark@chromium.org>