drm/imx: ipuv3-plane: disable local alpha for planes without alpha channel

Without this patch, after enabling the overlay plane with an RGBA
framebuffer, switching to a framebuffer without alpha channel would
cause the plane to vanish, since the pixel local alpha is constant
zero in that case. Disable local alpha again when setting an opaque
framebuffer.

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