drm: rockchip: set alpha_en to 0 if it is not used

alpha_en should be set to 0 if it is not used, i.e. to disable alpha
blending if it was enabled before and should be disabled now.

Fixes: 2aae8ed1

 ("drm/rockchip: Add per-pixel alpha support for the PX30 VOP")
Signed-off-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210528130554.72191-6-knaerzche@gmail.com