drm: rockchip: add alpha support for RK3036, RK3066, RK3126 and RK3188

With
commit 2aae8ed1

 ("drm/rockchip: Add per-pixel alpha support for the PX30 VOP")
alpha support was introduced for PX30's VOP.
RK3036, RK3066, RK3126 and RK3188 VOPs support alpha blending in the
same manner.
With the exception of RK3066 all of them support pre-multiplied alpha.

Signed-off-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210528130554.72191-5-knaerzche@gmail.com