drm/rockchip: Add per-pixel alpha support for the PX30 VOP

Compared to its predecessors, the PX30 VOP has a different register layout
for enabling per-pixel alpha. Instead of src_alpha_ctl and dst_alpha_ctl,
there is a single alpha control register. This register takes some fields
from src_alpha_ctl, but with a different layout.

Add support for the required fields to the PX30 VOP window descriptions,
which makes per-pixel-alpha formats behave correctly.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200416140526.262533-1-paul.kocialkowski@bootlin.com