drm/mxsfb: Disable overlay plane in mxsfb_plane_overlay_atomic_disable()

In the Linux kernel, the following vulnerability has been resolved: drm/mxsfb: Disable overlay plane in mxsfb_plane_overlay_atomic_disable() When disabling overlay plane in mxsfb_plane_overlay_atomic_update(), overlay plane's framebuffer pointer is NULL. So, dereferencing it would cause a kernel Oops(NULL pointer dereferencing). Fix the issue by disabling overlay plane in mxsfb_plane_overlay_atomic_disable() instead.