sdm: Make premultiplied as default blending

Make the enum for premultiplied as the first (and default) and
initialize layers with the same. Most of the layer formats used
commonly have an alpha channel, so having premultiplied as
default is beneficial from testing angle.

Change-Id: I8ffa82522c4d8b8cf8d39a8202a9daee04e7ea1e
1 file changed