drm: atmel-hlcdc: actually disable scaling when no scaling is required

The driver is only enabling scaling, but never disabling it, thus, if you
enable the scaling feature once it stays enabled forever.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: Alex Vazquez <avazquez.dev@gmail.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Fixes: 1a396789 ("drm: add Atmel HLCDC Display Controller support")
Cc: <stable@vger.kernel.org>