imx-drm: ipuv3-plane: allow local alpha in ipu_plane_mode_set()

For the overlay plane scanning out a framebuffer with an alpha component,
enable the DP local alpha feature on the partial plane.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>