drm/msm/mdp5: expose "alpha" property

We had this wired up already internally but initially did not expose the
property pending bikeshed about alpha and color management properties.
I noted that drm-hwc2 was looking for this property, and a couple other
drivers already support it in the same way.  So time to expose it!

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