Without the new blend modes("None", "Pre-multiplied" and "Coverage")
introduced in the below commit, the old userspace assumes alpha in
pixel is per-premultiplied by default. So, let's support the default
blend mode properly.
commit a5ec8332d4 ("drm: Add per-plane pixel blend mode property")
Signed-off-by: Liu Ying <victor.liu@nxp.com>