drm/rockchip: Disable blending for win0

Blending win0 with the background color doesn't seem to work
correctly. We only get the background color, no matter the contents of
the win0 framebuffer.  However, blending pre-multiplied color with the
default opaque black default background color is a no-op, so we can
just disable blending to get the correct result.

Signed-off-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Cc: Sandy Huang <hjc@rock-chips.com>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180418173152.93246-1-hoegsberg@chromium.org