drm: atmel-hlcdc: reset layer A2Q and UPDATE bits when disabling it

The A2Q (Add To Queue) and UPDATE bits are left in their previous state
when resetting the layer.
This lead to weird behavior when enabling the plane again: the framebuffer
previously queued is dequeued and we end up with access to an old memory
region.

Reset those bits when resetting the channel.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>