drm/fsl-dcu: fix alpha blending

Fix alpha blending by enabling alpha blending for the whole frame if
a color mode with alpha channel is selected (DRM_FORMAT_ARGB*). Also
support color modes without alpha channel (DRM_FORMAT_XRGB*) by just
not enabling alpha blending on layer level.

Signed-off-by: Stefan Agner <stefan@agner.ch>