drm_hwcomposer: set blending mode to kPreMult for GL output

The correct blending mode for the GL output is actually premult, because
regardless of the original layer blending mode, the result RGB components
are always pre-multiplied with alpha already.

This prevents the result of a SquashAll() from getting multiplied by
alpha twice.

TEST=verify the blinking is fixed on the status bar icons

Change-Id: Ie5bbc53110c342576b81818da6069cba201db609
1 file changed