drm: atmel-hlcdc: add a ->cleanup_fb() operation

Add a ->cleanup_fb() operation to avoid memory leaks when the atomic
operation is interrupted after the ->prepare_fb() call.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes 2389fc13 ("drm: atmel-hlcdc: Atomic mode-setting conversion")
Reviewed-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Nicolas Ferre <nicolas.ferre@atmel.com>