DRM_MODE_BLEND_PIXEL_NONE ignores an alpha channel.

Rcar-du driver supports only 3 formats with an alpha channel
(DRM_FORMAT_ARGB1555, DRM_FORMAT_ARGB8888 and DRM_FORMAT_ARGB4444). We
simply override the format passed to VSP1 for blending with the pixel
format without alpha channel.
Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Takanari Hayama <taki@igel.co.jp>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>