drm/imx: ipuv3-plane: add support for separate alpha planes

The IPUv3 can read 8-bit alpha values from a separate plane buffer using
a companion IDMAC channel driven by the Alpha Transparency Controller
(ATC) for the graphics channels. The conditional read mechanism allows
to reduce memory bandwidth by skipping reads of color data for
completely transparent bursts.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>