drm: atmel-hlcdc: Atomic mode-setting conversion

Convert the HLCDC driver to atomic mode-setting.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>