Merely writing zero to the CHn Source Image Format Register is not
sufficient to disable a plane, as the programmed register value is not
propagated immediately to the current side.  This can be seen when using
the -P option of modetest: the extra plane is displayed correctly, but
does not disappear after exit.

Fix this by doing the full update dance using the Blend Control
Register, like is done when enabling the plane.
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/54e2556e18b5524ce153a7ac79faf2c7b0a55260.1694767209.git.geert+renesas@glider.be