drm_hwcomposer: Use layer alpha to blend planes

Send the layer alpha to the drm plane's alpha property to
properly blend layers.

BUG=b/24821110
TEST=Tested on smaug with Downloads/About windows, blend properly

Change-Id: If96eb28d65d018863c39bc5a3554daef0264144b
Signed-off-by: Sean Paul <seanpaul@chromium.org>
3 files changed