drm: omapdrm: Reset the zorder property when disabling a plane

Whether to reset plane properties at disable time isn't well-defined in
DRM, but resetting only part of them is probably as bad as it can get.
Make the behaviour coherent by resetting the zorder property in addition
to the rotation property.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>