drm: atmel-hlcdc: move output mode selection in CRTC implementation

In order to support multiple outputs we need to move the output mode
selection to the CRTC object, so that the output validity check can be
done against the drm_atomic_state.

If the connectors selected by a specific mode setting are requiring
incompatible bus format the atomic operation is aborted (->atomic_check()
returns -EINVAL).

In order to implement that, we need to define our own CRTC state and
overload default ->reset(), ->atomic_duplicate_state() and
->atomic_destroy_state() functions.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Nicolas Ferre <nicolas.ferre@atmel.com>