Support upstream plane alpha property
This makes drm_hwc compatible with the upstream plane alpha property which uses 16 bits instead of 8.
I wanted to fixup the alpha in InitFromHwcLayer
too but realized it's only used from the no longer compiled HWC1 code
so it instead went out the window.
Tested on vc4, a good place to see plane alpha in action is the toast you get when hitting the build version in settings.