Support upstream plane alpha property

Stefan Schake requested to merge stschake/drm-hwcomposer:alpha-fixup into master

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.

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