SurfaceFlinger: Let HWC handle an alpha blending

                when either C2D or DYN is set

Before we only use GPU to blend an alpha value.
However C2D can also handle the alpha blending,
when the composition type is set to either C2D or DYN,
use HWC to handle this.
And set the alpha value of C2D to HWC_BLENDING_PREMULT,
so later C2D can set alpha blending parameter.

Change-Id: I53be1d4219fbf2467a6391276d59213858a50058